Chemical Calculations

Question 1

What is the relative mass of a proton?

Answer 1

1 AMU

Question 2

What is the relative mass of a neutron?

Answer 2

1 AMU

Question 3

What is the relative mass of an electron?

Answer 3

Almost 0, 1/2000

Question 4

What does the mass number mean?

Answer 4

The number of protons + the number of neutrons

Question 5

What is relative atomic mass?

Answer 5

The mass of an atom compared to 1/12 of an atom of carbon-12

Question 6

What is relative atomic mass measured in?

Answer 6

Grams

Question 7

What is the relative formula mass (Mr) of CO₂

Answer 7

12 + 16 + 16 = 44

Question 8

What is the relative formula mass (Mr) of MgO?

Answer 8

24 + 16 = 40

Question 9

Describe how to use apparatus to do a titration using 25 cm³ of dilute hydrochloric acid. In your answer you should include: • how you will determine the end point of the titration • how you will make sure the result obtained is accurate. [4]

Answer 9

-put sodium hydroxide (alkali) in the burette -add drop wise until the indicator turns pink -whilst swirling -read at the meniscus -read the start and end volumes

Question 10

Calcium carbonate reacts with hydrochloric acid as follows:

CaCO_3(s) + HCl_(aq) –> CaCl_2(aq) + CO_2(g) + H₂O_(l)

State what two changes you would observe.

Explain why the mass would decrease.

Answer 10

• Bubbles/Fizzing/Effervescence
• Calcium carbonate disappears/dissolves
• Mass decreases as a gas is made, which escapes the container.

Question 11

What is the equation to calculate moles from the mass of a substance?

Answer 11

moles = mass ÷ M_r

Question 12

What is the equation to calculate mass from the moles of a substance?

Answer 12

mass = M_r x moles

Question 13

How many atoms are in one mole of a substance (Avagadro’s constant)

Answer 13

6.02 x 10^{<span>23</span>}

Question 14

How many atoms are in one mole of CO₂?

Answer 14

6.02 x 10^{<span>23</span>}

Question 15

How many electrons are lost when one mole of sodium become sodium ions?

Answer 15

6.02 x 10²³ electrons

Question 16

How many moles are in 28g of iron?

Answer 16

moles = mass ÷ M_r

= 28 ÷ 56 = 0.5

Question 17

How many moles are in 69g of sodium?

Answer 17

Moles = mass ÷ Mr

= 69 ÷ 23 = 3

Question 18

How many moles are in 400g of CaCO₃?

Answer 18

moles = mass ÷ Mr

= 400 ÷ (40+12+16+16+16)

= 400 ÷ 100

=4

Question 19

How many moles are in 1kg of Fe₂O₃?

Answer 19

moles = mass ÷ Mr

1kg = 1000g

moles = 1000 ÷ (56+56+16+16+16)

=1000 ÷ 160

=6.25

Question 20

Nitrogen reacts with hydrogen to produce ammonia

N_2(g) + 3H_2(g) ⇌ 2NH_3(g)

Why might less ammonia be produced than expected?

Answer 20

• The reaction is reversible
• Not all of the nitrogen or hydrogen may react
• some of the ammonia may be lost when it is separated

Question 21

What is the equation to calculate percentage yield?

Answer 21

% Yield = Mass of product actually made

Maximum theoretical mass of product × 100