MOLES CALCULATIONS

Question 1

20g of sodium is burned in air. What mass of sodium oxide is formed?

Answer 1

Hint: 4Na + O2 –> 2Na2O

Question 2

What is the percentage of carbon in propene?

Answer 2

Hint: think about Ar

Question 3

What is the percentage calcium in calcium carbonate?

Answer 3

Hint: think about Ar

Question 4

33.6g of iron reacts with 14.4g of oxygen. What is the formula of iron oxide?

Answer 4

Hint: find the number of moles and divide both by the smaller number to get the ratio

Question 5

20g of calcium reacts with 35.5g of Cl. What is the formula of calcium chloride?

Answer 5

Hint: find the number of moles of both and divide both by the smaller number to find the ratio

Question 6

Which compound has 3g of carbon and 0.5g of hydrogen if the RMM is 42?

Answer 6

Hint: don’t be put off by them not telling you the name of the chemical produced. You know that it has both hydrogen and carbon in it and nothing else. So do the same as would to find the ratio of hydrogen to carbon (empirical formula). Once you’ve done that you can try and find a way to work out exactly what the compound is rather than just its empirical formula.

Question 7

Which compound has 48g of carbon and 12g of hydrogen if the RMM is 30?

Answer 7

Hint: RMM means relative molecular mass. Don’t be put off by them not telling you the name of the chemical produced. You know that it has both hydrogen and carbon in it and nothing else. So do the same as would to find the ratio of hydrogen to carbon (empirical formula). Once you’ve done that you can try and find a way to work out exactly what the compound is rather than just its empirical formula.

Question 8

6g of carbon is burned in air. What volume of carbon dioxide is formed?

Answer 8

Hint: one mole of any gas occupies 24 dm3 or 24,000 cm3

Question 9

90g of ethane is burned in air. What volume of carbon dioxide is formed?

Answer 9

Hint: formula of ethane is C2H6

Question 10

6.5g of zinc reacts with sulphuric acid. What volume of hydrogen is formed?

Answer 10

Hint: Zn + H2SO4 –> ZnSO4 + H2

Question 11

What is the mass of 2 litres of chlorine gas?

Answer 11

Hint: 1 litre = 1000 cm3

Question 12

In the electrolysis of molten sodium chloride, what mass and volume of chlorine are formed if 46g of sodium is deposited?

Answer 12

Hint: sodium chloride is NaCl
This question is no more difficult than working out what the equation is and looking at the ratio of Na to Cl2. Just remember that chlorine is a diatomic molecule.
2NaCl –> 2Na + Cl2

Question 13

In the electrolysis of aqueous sodium chloride, what mass and volume of hydrogen are formed if 142g of chlorine are discharged?

Answer 13

Hint: this question is about knowing the half equations happening at each electrode:
2H+ + 2e- –> H2
2Cl- –> Cl2 + 2e-

The overall equation isn’t necessarily needed but here it is anyway: 2NaCl + 2H2O –> Cl2 + H2 2NaOH.

Question 14

What mass of magnesium oxide will be formed when 10g of magnesium is burned in air?

Answer 14

Hint: 2Mg + O2 –> 2MgO

Question 15

This is a much tougher one. One that I’m not sure we have covered and I’m not sure will come up BUT we did do some work on Faradays.

The electrolysis of molten lead bromide - PbBr2
produces lead at the cathode and bromine at the anode.
A current of 10 amps is allowed to flow
through molten lead bromide for 5 hours.
What mass of lead is deposited at the cathode?

Answer 15

Hint: One mole of electrons is called a Faraday

1 Faraday = 96,500 Coulombs

Coulombs are just units of electrical charge and are represented by the letter Q. To find the Coulombs you need the equation:

Q = I x t
where I is current in amps and t is time in seconds.

So from this you can work out the Coulombs. From that you can work out the Faradays, and the rest is just ratios using the half equations, as with all the rest of our calculations. Give it a try. Not a problem if you struggle.

Question 16

This is another hard one. But one that with a bit of time you should be able to work out, and if you do you will be well on your way to being really happy with moles calculations:

In a titration, 50 cm3 of 2 mol/dm3 sodium hydroxide
was exactly neutralised by 30 cm3 of hydrochloric acid.
What is the concentration of the hydrochloric acid in mol/dm3?

Answer 16

Hint: The first thing we need to do is work out the equation of the neutralisation reaction:

NaOH + HCl –> NaCl + H2O

From this we know the ratio of the NaOH to HCl so using the other triangle we can work out the number of moles of each. Do you remember the other triangle?:

Moles = Concentration x Volume

So as with a lot of our calculations, we first work out the moles of one chemical, therefore then work out the moles of the second chemical, and from this work out the concentration of the second chemical, using the same triangle.