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Understand Moles

Worksheet Overview

QUESTION 1 of 10

Moles sometimes sound scary. They aren't.

When chemists talk about moles, they don't mean the animal. Moles are the way that chemists count collections of atoms or molecules.

A mole is really just the name for a big number. It has to be a big number, because individual atoms are so small. In the same way that people buy eggs in boxes of six, chemists count atoms (or molecules, or electrons) in batches of 602 200 000 000 000 000 000 000. In standard form, this number is 6.022 x 1023. It's also called Avagadro's number.

1 mole of something = 6.022 x 1023 of something.

This number might seem odd, but it has three useful properties.

If you look up the relative atomic mass, Ar, of an element, the mass of 1 mole of atoms of that element will be Ar grams.

The relative atomic mass of Fe is 56. If you have 56 g of Fe, there are 1 mole of atoms or 6.022 x 1023 atoms.

If you work out the relative formula mass, Mr, of a substance, the mass of 1 mole of that substance will be Mr grams.

Water is H2O, so its relative formula mass, Mr = (2 x 1) + 16 = 18.

If you have 18 g water, there are 1 mole of water molecules, or 6.022 x 1023 molecules.

If you have a 1 mole of gas molecules at room temperature and pressure, their volume will be 24 000 cm3 (or 24 litres).

This works whatever the gas is.

Converting between mass and moles

The equation for this is:

number of moles = real mass in g ÷ relative formula mass

For example, how many moles are there in 12 g of sodium hydroxide, NaOH?

1. Work out the relative formula mass: Mr = 23 + 16 + 1 = 40.

2. Substitute into the equation: number of moles = 12 ÷  40 = 0.3.

Using moles in reaction equations

Moles are a good way to think about the ratios in chemical reactions. For example, sodium hydroxide reacts with hydrochloric acid to make sodium chloride + water:

NaOH (aq) + HCl (aq) →NaCl (aq) + H2O (l)

Suppose we have 12 g of sodium hydroxide. We've already worked out that is 0.3 moles of NaOH. According to the reaction equation, 1 mole of NaOH plus 1 mole of HCl react to make 1 mole of NaCl plus 1 mole of water. So if we have 0.3 moles of NaOH, we have 0.3 moles of everything else.

The formula mass of NaCl = 23 + 35.5 = 58.5, so 1 mole of NaCl is 58.5 g.

Therefore the mass of NaCl made is 0.3 x 58.5 g = 17.55 g.

You don't have to do this problem using moles- you can work out the ratios from scratch each time. But it's harder that way. The reason moles are useful is because they give the ratio a name, which makes it easier to think about.

Avagadro's number is... 

(tick all the correct answers)

the number of H2 molecules in 2 g of H.

the number of particles in a mole of particles

6.022 x 1023

The number of He atoms in 4 g of He

What is the volume of 1 mol of gas?

(Tick all the correct answers.)

24 litres

24 000 litres

24 000 cm3

24 cm3

What is the mass in grams of 3 moles of LiF?

What is the mass in grams of 0.4 moles of CO2?

How many moles are there in 9.2 g of NO2?

How many moles are there in 1.6 g of oxygen O2?

What is the volume of this amount of gas?

If you burn magnesium in air, it reacts with oxygen to make magnesium oxide.

The balanced symbol equation is

2 Mg (s) + O2 (g) → 2 MgO (s)

If we start with 2.4 g Mg, how many moles of MgO will we make? What mass of MgO is this?

If you burn magnesium in air, it reacts with oxygen to make magnesium oxide.

The balanced symbol equation is

2 Mg (s) + O2 (g) → 2 MgO (s)

If we start with 2.4 g Mg, that is 0.1 moles. How many moles of O2 gas will be needed to react completely with the magnesium, and what is the mass and volume of this gas?

If we react calcium with hydrochloric acid, the balanced reaction is

Ca (s) + 2 HCl (aq) → CaCl2 (aq) + H2 (g)

If we react 2.4 g calcium, how much H2 gas will we make, in moles, in grams and in cm3?

If we react calcium with hydrochloric acid, the balanced reaction is

Ca (s) + 2 HCl (aq) → CaCl2 (aq) + H2 (g)

If we react 1.6 g calcium, how much CaCl2 will we make, in moles and in grams?

  • Question 1

Avagadro's number is... 

(tick all the correct answers)

CORRECT ANSWER
the number of H2 molecules in 2 g of H.
the number of particles in a mole of particles
6.022 x 1023
EDDIE SAYS
The two definitions of Avagadro's number are correct. The relative formula mass of H2 is 2, so 2 g = 1 mole = 6.022 x 1023 molecules. The relative atomic mass of He is 4, so 4 g = 1 mole.
  • Question 2

What is the volume of 1 mol of gas?

(Tick all the correct answers.)

CORRECT ANSWER
24 litres
24 000 cm3
EDDIE SAYS
This number is important, so make sure you learn it by heart. You will sometimes see it written as 24 dm3. A dm is 10 cm, so a dm3 is a box 10 cm x 10 cm x 10 cm, which gives 1 litre.
  • Question 3

What is the mass in grams of 3 moles of LiF?

CORRECT ANSWER
78
EDDIE SAYS
The relative atomic masses are Li: 7, F: 19, so the relative formula mass of LiF = 26. That means 1 mole LiF has mass = 26 g, so 3 moles = 3 x 26 = 78 g.
  • Question 4

What is the mass in grams of 0.4 moles of CO2?

CORRECT ANSWER
17.6
EDDIE SAYS
The relative atomic masses are C: 12, O: 16, so the relative formula mass of CO2 = 12 + (2 x 16) = 44. That means 1 mole CO2 has mass = 44 g, so 0.4 moles = 0.4 x 44 = 17.6 g.
  • Question 5

How many moles are there in 9.2 g of NO2?

CORRECT ANSWER
0.2
EDDIE SAYS
The relative atomic masses are N = 14, O = 16, so the relative formula mass of NO2 = 14 + (2 x 16) = 46. 1 mole of NO2 is 46 g, so 9.2 g is... number of moles = 9.2 ÷ 46 = 0.2 mol
  • Question 6

How many moles are there in 1.6 g of oxygen O2?

What is the volume of this amount of gas?

CORRECT ANSWER
EDDIE SAYS
The relative atomic mass of O = 16, so the relative formula mass of O2 = (2 x 16) = 32. Number of moles = 1.6 ÷ 32 = 0.05, so Gas volume = 0.05 x 24 000 = 1200 cm3
  • Question 7

If you burn magnesium in air, it reacts with oxygen to make magnesium oxide.

The balanced symbol equation is

2 Mg (s) + O2 (g) → 2 MgO (s)

If we start with 2.4 g Mg, how many moles of MgO will we make? What mass of MgO is this?

CORRECT ANSWER
EDDIE SAYS
The relative atomic masses are Mg = 24, O = 16. So the formula mass of MgO = 24 + 16 = 40. That means 1 mole of Mg = 24 g, and 1 mole of MgO = 40 g. We only have 2.4 g of Mg, and 2.4 ÷ 24 = 0.1, so we have 0.1 mole of Mg, and make 0.1 mole of MgO. That means that the mass of MgO = 0.1 x 40 g = 0.4 g.
  • Question 8

If you burn magnesium in air, it reacts with oxygen to make magnesium oxide.

The balanced symbol equation is

2 Mg (s) + O2 (g) → 2 MgO (s)

If we start with 2.4 g Mg, that is 0.1 moles. How many moles of O2 gas will be needed to react completely with the magnesium, and what is the mass and volume of this gas?

CORRECT ANSWER
EDDIE SAYS
In the balanced equation, 2 moles of magnesium atoms react with 1 mole of oxygen molecules. So if we have 0.1 moles of magnesium atoms, we need 0.05 moles of oxygen molecules. 1 mole of gas has a volume of 24 000 cm3, so 0.05 moles has volume 0.05 x 24 000 = 1200 cm3.
  • Question 9

If we react calcium with hydrochloric acid, the balanced reaction is

Ca (s) + 2 HCl (aq) → CaCl2 (aq) + H2 (g)

If we react 2.4 g calcium, how much H2 gas will we make, in moles, in grams and in cm3?

CORRECT ANSWER
EDDIE SAYS
The relative atomic mass of Ca is 40, so 2.4 g is 2.4 ÷ 40 = 0.06 moles. The relative formula mass of H2 is (2 x 1) = 2. Since 1 mole of H2 is 2 g, 0.06 moles = 0.06 x 2 = 0.12 g. The volume = 0.06 x 24 000 = 1440 cm3
  • Question 10

If we react calcium with hydrochloric acid, the balanced reaction is

Ca (s) + 2 HCl (aq) → CaCl2 (aq) + H2 (g)

If we react 1.6 g calcium, how much CaCl2 will we make, in moles and in grams?

CORRECT ANSWER
EDDIE SAYS
The relative atomic mass of Ca is 40, so 1.6 g is 1.6 ÷ 40 = 0.04 moles. The relative formula mass of CaCl2 is 40 + (2 x 35.5) = 111. Since 1 mole of CaCl2 is 111 g, 0.04 moles = 0.04 x 111 = 4.44 g. The volume = 0.06 x 24 000 = 1440 cm3
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