Combined 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 %