chapter 3 Atoms and molecules

Explanation:

In order to verify that the law of conservation of mass is upheld, it is necessary to determine the total mass of the reactants and products and compare them. This law states that the total mass of the reactants must always equal the total mass of the products.

The chemical equation for the reaction between acetic acid and sodium carbonate is as follows:

Sodium carbonate(Na₂CO₃) + acetic acid(CH₃COOH) → Sodium acetate(CH₃COONa) + carbon dioxide(CO₂) +  water(H₂O)

And from the question the mass of reactants and products are:

Sodium carbonate(Na₂CO₃) = 5.3g

Acetic acid(CH₃COOH)= 6g

Sodium acetate(CH₃COONa)= 8.2g

Carbon dioxide(CO₂)= 2.2g

Water(H₂O)= 0.9g

According to law of conservation of mass

Total mass of reactants: 5.3 g + 6 g = 11.3 g

The total mass of the products: 2.2 g + 0.9 g + 8.2 g = 11.3 g

As we can see, the total mass of the products is the same as the total mass of the reactants.

Hence, the observations are in agreement with the law of conservation of mass.

Explanation:

The ratio in which hydrogen and oxygen combine to form water is 1:8. 

This means that for every 1 gram of hydrogen, 8 grams of oxygen are required to form water.

Let's calculate how much oxygen would be required to react completely with 3 g of hydrogen gas:

Mass of oxygen required = (8/1) 3 g = 24 g

Therefore, 24 g of oxygen gas would be required to react with 3 g of hydrogen gas to form water.

Explanation:

The law of conservation of mass indicates that in a chemical reaction, mass cannot be created or destroyed. This principle aligns with the first postulate of Dalton's atomic theory, which proposes that elements are composed of tiny, indivisible particles called atoms that cannot be created, destroyed, or transformed into other atoms.

According to Dalton's theory, during a chemical reaction, atoms are rearranged to form new compounds, but the overall number of atoms in the reactants and products remains constant, and no atoms are created or destroyed. This is in agreement with the law of conservation of mass, which requires the total mass of the reactants to be equal to the total mass of the products.

As a result, the first postulate of Dalton's atomic theory is a consequence of the law of conservation of mass.

Explanation:

The law of definite proportions can be explained by the second postulate of Dalton's atomic theory. This law states that a specific chemical compound always contains the same proportion of elements by mass. In other words, the ratio of the masses of the elements in a compound remains constant, regardless of the method or source of preparation.

According to the second postulate of Dalton's atomic theory, atoms of different elements combine in simple whole-number ratios to form compounds. For instance, a compound AB2 may result from the combination of one atom of element A and two atoms of element B.

As atoms of various elements have distinct masses, the ratio of the masses of the elements in a compound is always a simple whole-number ratio, which is determined by the ratio of the numbers of atoms in the compound. Therefore, the second postulate of Dalton's atomic theory, which explains that atoms combine in simple whole-number ratios to form compounds, can account for the law of definite proportions.

Explanation:

The atomic mass unit (amu) is a unit of measurement used to describe the mass of atoms and molecules. It is defined as precisely 1/12th of the mass of an atom of carbon-12. This means that one atomic mass unit is equivalent to 1/12th of the mass of a single carbon-12 atom, which is assigned a mass of 12 atomic mass units. The atomic mass unit is also equivalent to1.66 10^-27kg.

Explanation:

It is impossible to see an atom with the naked eye because atoms are extremely small.

The size of an atom ranges from 0.1 to 0.5 nanometers (nm), which is approximately 10,000 times smaller than the width of a human hair. Since the wavelength of visible light is much larger than the size of an atom, an optical microscope that relies on visible light cannot produce an image of an atom.

Explanation:

a) The formula for the chemical compound 

    Sodium oxide is Na₂O

b) The formula for the chemical compound

    Aluminium chloride is AlCl₃

c) The formula for the chemical compound 

    Sodium sulphide is Na₂S

d) The formula for the chemical compound

    Magnesium hydroxide is Mg(OH)₂

Explanation:

 a) The name of the chemical compound

    Al₂(SO₄)₃ is Aluminium sulphate.

b) The name of the chemical compound

   CaCl₂ is Calcium chloride.

c) The name of the chemical compound

   K₂SO₄is Potassium sulphate.

d) The name of the chemical compound

   KNO₃ is Potassium nitrate.

e) The name of the chemical compound

   CaCO₃ is Calcium carbonate.

Explanation:

A chemical formula is a concise representation of a chemical compound that conveys information about the atoms in the compound. It is a shorthand method for describing the composition of a substance by using chemical symbols and numerical subscripts. The chemical symbols in a formula indicate the elements present in the compound, while the numerical subscripts show the relative number of atoms of each element. For example, the chemical formula H2O represents water, and it indicates that a molecule of water comprises two hydrogen atoms (H) and one oxygen atom (O).

Explanation:

a) The no. of atoms present in the molecule H₂S can be calculated as

   H₂S contains hydrogen which has a subscript of “2” which means it has 2 atoms of hydrogen and only 1 atom of sulphur.

Total  2+1=  3 atoms.

b) The number of atoms present in the ion PO₄^3-can be calculated as

   PO₄^3-contains 1 atom of phosphorus and 4 atoms of oxygen.

Total 1+4= 5 atoms.

Explanation:

a) The molecular mass of H₂ can be calculated as

   2 atomic mass of H = 21=2u

b) The molecular mass of O₂ can be calculated as

   2 atomic mass of O = 216=32u

c) The molecular mass of Cl₂ can be calculated as

   2 atomic mass of Cl = 235.5=71u

d) The molecular mass of CO₂  can be calculated as

   atomic mass of C + 2 atomic mass of O = 12+(216)=44u

e) The molecular mass of CH₄  can be calculated as

   atomic mass of C + 4 atomic mass of H = 12+(41)=16u 

f) The molecular mass of C₂H₆ can be calculated as

   2 atomic mass of C + 6 atomic mass of H = (212)+(61)=30u

g) The molecular mass of C₂H₄can be calculated as

   2 atomic mass of C + 4 atomic mass of H = (212)+(41)=28u

h) The molecular mass of NH₃can be calculated as

   atomic mass of N + 3 atomic mass of H = 14+(31)=17u

i) The molecular mass of CH₃OH can be calculated as

   atomic mass of C + 3 atomic mass of H + atomic mass of O + atomic mass of H = 12+(31)+16+1=32u

Explanation:

To calculate the formula unit mass, we need to add the atomic masses of all the atoms present in one formula unit of the compound.

For ZnO:

ZnO contains 1 atom of Zn and 1 atom of O.

Formula unit mass of ZnO = Atomic mass of Zn + Atomic mass of O

= 65u + 16u

= 8 u

For Na₂O:

Na₂O contains 2 atoms of Na and 1 atom of O.

Formula unit mass of Na₂O = 2   Atomic mass of Na + Atomic mass of O

= (2   23u) + 1 u

= 62u

For K₂CO₃:

K₂CO₃ contains 2 atoms of K, 1 atom of C, and 3 atoms of O.

Formula unit mass of K₂CO₃= 2   Atomic mass of K + Atomic mass of C + 3   Atomic mass of O

= (2   39u) + 1u + (3   16u)

= 138u

Therefore, the formula unit masses of ZnO, Na₂O, and K₂CO₃are 81u, 62u, and 138u, respectively.

Explanation:

If one mole of carbon atoms weighs 12 grams, then the molar mass of carbon is 12 g/mol. This means that 1 mole of carbon atoms contains Avogadro's number (6.022*10²³) of carbon atoms and weighs 12 grams.

To find the mass of one carbon atom, we can divide the molar mass of carbon by Avogadro's number:

Mass of 1 carbon atom = Molar mass of carbon / Avogadro's number

Mass of 1 carbon atom = 12/6.022*10²³

Mass of 1 carbon atom ≈ 1.99*10^-23 grams

Therefore, the mass of one atom of carbon is 1.99*10^-23grams.

Explanation:

To compare the number of atoms in 100 grams of sodium and 100 grams of iron, we need to first calculate the number of moles of each element present in the given mass:

Number of moles of sodium = Mass of sodium / Atomic mass of sodium

= 100 g / (23 g/mol)

= 4.35 mol

Number of moles of iron = Mass of iron / Atomic mass of iron

= 100 g / (56 g/mol)

= 1.79 mol

Next, we can use Avogadro's number to find the number of atoms in each case:

Number of atoms of sodium = Number of moles of sodium   Avogadro's number

= 4.35 mol   6.022*10²³ atoms/mol

= 26.18*10²³ atoms

Number of atoms of iron = Number of moles of iron  Avogadro's number

= 1.79 mol   6.022*10²³ atoms/mol

= 10.75*10²³ atoms

Therefore, 100 grams of sodium has more atoms than 100 grams of iron, as it contains approximately  26.18*10²³ atoms compared to 10.75*10²³ atoms in 100 grams of iron.

Explanation:

To calculate the percentage composition by weight of the compound, we need to determine the mass of each element present in the compound and then divide by the total mass of the compound and multiply by 100%.

Now we can calculate the percentage composition of each element in the compound:

Percentage of boron by weight = (Mass of boron / Total mass of the compound) 100%

= 0.0960/24g*100%

= 40%

Percentage of oxygen by weight = (Mass of oxygen / Total mass of the compound) 100%

=0.1440/24g*100%

= 60%

Therefore, the compound is composed of 40% boron and 60% oxygen by weight.

Explanation:

Using the given data, we can calculate the amount of carbon and oxygen required to produce 11.00 g of carbon dioxide:

The balanced chemical equation for the combustion of carbon in oxygen is:

C+O₂ =CO₂

Amount of carbon = Mass of carbon / Molar mass of carbon

= 3.0 g / (12.0 g/mol)

= 0.25 mol

Amount of oxygen = Mass of oxygen / Molar mass of oxygen

= 8.00 g / (16.00 g/mol)

= 0.50 mol

Since we have 0.50 mol of oxygen available, it is in excess and will not be completely used up. 

Only 0.25 mol of oxygen will react with 0.25 mol of carbon to produce 0.25 mol of carbon dioxide.

Therefore, the amount of carbon dioxide produced will be limited by the amount of carbon present.

Using the stoichiometric ratio, we can calculate the amount of carbon dioxide produced from 0.25 mol of carbon:

Amount of carbon dioxide = Amount of carbon (in mol) Molecular mass of carbon dioxide

= 0.25 mol 44.00 g/mol

= 11.00 g

Therefore, 11.00 g of carbon dioxide will be produced when 3.00 g of carbon is burnt in 50.00 g of oxygen.

The law of definite proportions will govern the answer to this question, which states that the ratio of the masses of two elements in a compound is always the same, regardless of the size or source of the compound.

Explanation:

Polyatomic ions are groups of two or more atoms that carry an overall electric charge and act as a single charged entity in a chemical reaction. They are formed when atoms of different elements bond covalently or ionically to form a molecule that has a net charge due to the loss or gain of electrons.

Examples of common polyatomic ions include:

1. Nitrate ion (NO^3-)

2. Carbonate ion (CO₃^2-)

3. Hydroxide ion (OH^-)

4. Ammonium ion (NH⁴⁺)

5. Sulphate ion (SO₄^2-)

6. Phosphate ion (PO₄^3-)

7. Acetate ion (C₂H₃O^2-)

8. Cyanide ion (CN^-)

Polyatomic ions are often found in ionic compounds, such as salts and acids, and play important roles in many chemical reactions.

Explanation:

a) The chemical formula for the compound

    Magnesium chloride is MgCl₂

b) The chemical formula for the compound

    Calcium oxide is CaO

c) The chemical formula for the compound

    Copper nitrate is Cu(NO₃)₂

d) The chemical formula for the compound

    Aluminium chloride is AlCl₃

e) The chemical formula for the compound

    Calcium carbonate is CaCO₃

Explanation:

a) The elements present in the compound quicklime are calcium and oxygen and it is denoted by CaO.

b) The elements present in the compound hydrogen bromide are hydrogen and bromine and it is denoted by HBr.

c) The elements present in the compound baking powder are sodium, carbon, hydrogen and oxygen and it is denoted by NaHCO_3.

d) The elements present in the compound potassium sulphate are sulphur, oxygen and potassium and it is denoted by K₂SO₄.

Explanation:

a) The molar mass of ethyne (C₂H₂) can be calculated by adding up the atomic masses of the constituent atoms:

Molar mass of C₂H₂= (2 atomic mass of C) + (2 atomic mass of H)

= (2 12 g/mol) + (2 1 g/mol)

= 26 g/mol

b) The molar mass of sulphur molecule (S₈) can be calculated by multiplying the atomic mass of sulphur by 8:

Molar mass of S8 = 8 atomic mass of S

= 8 32 g/mol

= 256 g/mol

c) The molar mass of phosphorus molecule (P₄) can be calculated by multiplying the atomic mass of phosphorus by 4:

Molar mass of P₄ = 4 atomic mass of P

= 4 31 g/mol

= 124 g/mol

d) The molar mass of hydrochloric acid (HCl) can be calculated by adding up the atomic masses of the constituent atoms:

Molar mass of HCl = atomic mass of H + atomic mass of Cl

= 1 g/mol + 35.5 g/mol

= 36.5 g/mol

e) The molar mass of nitric acid (HNO₃) can be calculated by adding up the atomic masses of the constituent atoms:

Molar mass of HNO₃ = atomic mass of H + atomic mass of N + (3 atomic mass of O)

= 1 g/mol + 14 g/mol + (3 16 g/mol)

= 63 g/mol

Explanation:

To calculate the mass of a certain number of moles of a substance, we can use the formula:

mass = molar mass number of moles

a) The molar mass of nitrogen (N) is 14 g/mol (atomic mass of N). Therefore, the mass of 1 mole of nitrogen atoms is:

mass = molar mass number of moles

= 14 g/mol 1 mol

= 14 g

b) The molar mass of aluminium (Al) is 27 g/mol (atomic mass of Al). Therefore, the mass of 4 moles of aluminium atoms is:

mass = molar mass number of moles

= 27 g/mol 4 mol

= 108 g

c) The molar mass of sodium sulphite (Na2SO3) is 126 g/mol (2 atomic mass of Na + atomic mass of S + 3 atomic mass of O). So, the mass of 10 moles of sodium sulphite is:

mass = molar mass number of moles

= 126 g/mol 10 mol

= 1260 g

Explanation:

To convert a given mass of a substance to the number of moles, we can use the formula:

number of moles = given mass / molar mass

(a) The molar mass of oxygen (O₂) is 32 g/mol. Therefore, the number of moles of 12 g of oxygen gas is:

number of moles = given mass / molar mass

= 12 g / 32 g/mol

= 0.375 mol

Therefore, 12 g of oxygen gas is equivalent to 0.375 moles.

(b) The molar mass of water (H₂O) is 18.02 g/mol. Therefore, the number of moles of 20 g of water is:

number of moles = given mass / molar mass

= 20 g / 18.02 g/mol

= 1.11 mol

Therefore, 20 g of water is equivalent to 1.11 moles.

(c) The molar mass of carbon dioxide (CO₂) is 44 g/mol. Therefore, the number of moles of 22 g of carbon dioxide is:

number of moles = given mass / molar mass

= 22 g / 44 g/mol

= 0.5 mol

Therefore, 22 g of carbon dioxide is equivalent to 0.5 moles.

Explanation:

(a) The molar mass of oxygen (O₂) is 16 g/mol (atomic mass of O). Therefore, the mass of 0.2 mole of oxygen atoms is:

mass = molar mass number of moles

= 16 g/mol 0.2 mol

= 3.2 g

Therefore, the mass of 0.2 mole of oxygen atoms is 3.2 g.

(b) The molar mass of water (H₂O) is 18.02 g/mol. Therefore, the mass of 0.5 mole of water molecules is:

mass = molar mass number of moles

= 18 g/mol 0.5 mol

= 9 g

Therefore, the mass of 0.5 mole of water molecules is 9 g.

Explanation:

To calculate the number of molecules of S₈ present in 16 g of solid sulphur, we need to first convert the given mass of sulphur to the number of moles using its molar mass, and then use Avogadro's number to convert the number of moles to the number of molecules.

The molar mass of S8 is 256 g/mol (8 atomic mass of S). So, the number of moles of sulphur in 16 g of solid sulphur is:

number of moles = given mass / molar mass

= 16 g / (256 g/mol)

= 0.0625 mol

We can then convert the number of moles to the number of molecules of S8 using Avogadro's

number, which is 6.022*10²³ molecules/mol:

number of molecules = number of moles Avogadro's number

= 0.0625 mol * 6.022*10²³ molecules/mol

= 3.76 * 10²² molecules

Therefore, there are approximately 3.76 *10²² molecules of S8 present in 16 g of solid sulphur.

Explanation:

To calculate the number of aluminium ions present in 0.051 g of aluminium oxide, we first need to determine the number of moles of aluminium oxide. We can then use the balanced chemical equation for the formation of aluminium oxide to determine the number of moles of aluminium ions present in the given mass of aluminium oxide.

The molar mass of aluminium oxide (Al₂O₃) is 102 g/mol. Therefore, the number of moles of aluminium oxide in 0.051 g of aluminium oxide is:

number of moles = given mass / molar mass

= 0.051 g / 102 g/mol

= 0.0005 mol

According to the balanced chemical equation for the formation of aluminium oxide from aluminium and oxygen:

4Al+3O2₂= 2Al₂O₃

We know that for every 2 moles of Al₂O₃ formed, we need 4 moles of aluminium ions. Therefore, the number of moles of aluminium ions present in 0.0005 mol of aluminium oxide is:

number of moles of Al ions = (4/2) number of moles of Al₂O₃

= 2 0.0005 mol

= 0.001 mol

Finally, we can calculate the number of aluminium ions present in 0.051 g of aluminium oxide using the number of moles of aluminium ions and Avogadro's number 6.0221023 ions/mol):

number of aluminium ions = number of moles of Al ions x Avogadro's number

= 0.001 mol 6.0221023 ions/mol

= 6.0221020ions

Therefore, there are approximately 6.022*10²⁰ aluminium ions present in 0.051 g of aluminium oxide.