Separate Chemistry - 5.3

Question 1

What does the law of the conservation of mass state?

Answer 1

No atoms are lost or made during a chemical reaction

Question 2

A piece of magnesium is added to acid. What would happen to the mass reading on the balance and why?

Answer 2

It would stay the same because there is a lid on the conical flask so no gas can escape.

Question 3

Count the number of elements in the following compounds:

a) CaO
b) Al₂O₃
c) H₂SO₄

Answer 3

a) CaO - 2 elements (Ca and O)
b) Al₂O₃ - 2 elements (Al and O)
c) H₂SO₄ - 3 elements (H, S and O)

Question 4

Count the number of atoms in the following compounds:

a) CaO
b) Al₂O₃
c) H₂SO₄

Answer 4

a) CaO - 2 atoms (1 x Ca and 1 x O)
b) Al₂O₃ - 5 atoms (2 x Al and 3 x O)
c) H₂SO₄ - 7 atoms (2 x H, 1 x S and 4 x O)

Question 5

Calculate the relative formula mass (Mr) of each of the compounds (a, b & c) below.

AR: Mg = 24; Cl = 35.5; Ca = 40; O = 16; Al = 27

a) CaO
b) MgCl₂
c) Al₂O₃

*this is just an example, you need to be able to do this for any compound.

Answer 5

a) CaO: 40 + 16 = 56
b) MgCl₂: 24 + (35.5x2) = 95
c) Al₂O₃: (27x 2) + (16 x 3) = 102

Question 6

A piece of magnesium is added to acid. What would happen to the mass reading on the balance and why?

Answer 6

It would decrease because there is no lid on the conical flask so gas can escape and therefore mass is ’lost’ from the flask.

Question 7

Calculate the uncertainty in the following sets of results

a) 1, 3, 6, 7, 10
b) 22, 25, 26, 25, 24

*this is just an example, you need to be able to do this for any set of data given.

Answer 7

Use the equation: uncertainty = range / 2

a) 1, 3, 6, 7, 10

Range = 10 – 1 = 9. Uncertainty = 9/2

Uncertainty = ± 4.5

b) 22, 25, 26, 25, 24

Range = 26 – 22 = 4. Uncertainty = 4/2

Uncertainty = ± 2

Question 8

Calculate the percentage mass of hydrogen in:

Ar: N = 14 ; H = 1 ; S = 32 ; O = 16.

a) NH₃
b) H₂SO₄

Give your answer to the nearest whole number.

*this is just an example, you need to be able to do this for any compound when given the appropriate information.

Answer 8

% mass = (Ar x number of atoms of the element) / (Mr of compound) x 100

a) NH₃ : (3x1) / 17 = 0.176
0. 176 x 100 = 18 %
b) H₂SO₄ : (2x1) / 98
0. 02 x 100 = 2 %

Question 9

Separate Q. Give three reasons why you might not achieve the amount of product you have calculated (theoretical yield).

Answer 9

• The reaction may not go to completion because it is reversible.
• Some of the product may be lost when it is separated from the reaction mixture.
• Some of the reactants may react in ways different to the expected reaction.

Question 10

Separate Q. What does ‘yield’ mean?

Answer 10

The amount of product that is obtained

Question 11

Separate Q. What equation would you use to work out the percentage yield of a reaction?

Answer 11

% yield=(actual yield)/(theoretical yield) x 100

Question 12

Separate Q. What does atom economy mean and why is it important?

Answer 12

• Atom economy is a measure of the amount of starting materials that end up as useful products.
• It’s important for sustainable development and economical reasons.

Question 13

Separate Q. How would you calculate percentage atom economy?

Answer 13

% atom economy = (Mr of desired product) / (Sum Mr of all reactants) x 100