Relative formula mass and the mole

Science · Matter & Materials · ages 15-16

Name ______________________ Date ____________

A question gives you a number of moles and a relative formula mass, and asks for a mass in grams. What do you do?
- divide the moles by the Mr
- divide the Mr by the number of moles
- add the moles to the Mr
- multiply the moles by the Mr
How many particles are there in one mole of any substance?
- one, since a mole is a single particle
- about 6.02 x 10^23, whatever the substance
- it depends entirely on the relative formula mass
- exactly one gram's worth of them
Calcium chloride is CaCl₂, with Ca = 40 and Cl = 35.5. What is its relative formula mass?
- 111
- 75.5
- 151
- 71
Lithium fluoride is LiF, with Li = 7 and F = 19. What is its relative formula mass?
- 7
- 19
- 26
- 12
Two jars each hold 10 g of powder. One is magnesium oxide and one is calcium carbonate. Do they hold the same number of particles?
- no, one unit of each weighs a different amount
- yes, because the two masses are the same
- yes, because a gram is a fixed number of particles
- no, because one jar happens to be fuller than the other
One mole of water and one mole of carbon dioxide weigh the same.
Circle one: True False
Calculate the relative formula mass of Mg(OH)2. Use Mg = 24, O = 16, H = 1.
Answer: ______________
Magnesium nitrate is Mg(NO₃)₂, with Mg = 24, N = 14 and O = 16. What is its relative formula mass?
Answer: ______________
Ammonium sulfate is (NH₄)₂SO₄, with N = 14, H = 1, S = 32 and O = 16. What is its relative formula mass?
- 114
- 96
- 132
- 128
Which holds more molecules: 88 g of carbon dioxide, Mr 44, or 54 g of water, Mr 18?
- the carbon dioxide, since its mass is larger
- the water, since 54 g of it comes to three moles
- they hold the same number, since both are pure
- the carbon dioxide, since its molecules are heavier

Answer key

For grown-ups. Fold this page away before handing over the rest.

Relative formula mass and the mole W1-mt_-SiMKJK4rH-s1

multiply the moles by the Mr · Each mole weighs Mr grams, so a number of moles weighs that many lots of Mr. Repeated lots of a number is a multiplication.
about 6.02 x 10^23, whatever the substance · A mole is a fixed count, like a dozen but far larger. The substance changes what that count weighs, never the count itself.
111 · The subscript 2 applies to the chlorine only: 40 + 2 x 35.5 = 40 + 71 = 111. 71 on its own is the two chlorines with the calcium left out.
26 · 7 + 19 = 26. A formula mass adds the atoms together; it never subtracts one from the other.
no, one unit of each weighs a different amount · Grams measure mass, not count. A heavier particle means fewer of them fit into the same mass, so equal masses of substances with different formula masses hold unequal counts.
False · They hold the same number of molecules, but a carbon dioxide molecule is heavier. A mole of water is 18 g and a mole of carbon dioxide is 44 g.
58 · Mg is 24, each OH is 17, and two of them give 24 + 34 = 58.
148 · One NO₃ is 14 + 3 x 16 = 62. The bracket is taken twice, so 2 x 62 = 124, and 24 + 124 = 148.
132 · One NH₄ is 14 + 4 = 18, and the bracket is doubled, giving 36. SO₄ is 32 + 64 = 96. Then 36 + 96 = 132.
the water, since 54 g of it comes to three moles · Turn both into moles before comparing. 88 / 44 = 2 moles of carbon dioxide, and 54 / 18 = 3 moles of water, so the water holds more molecules despite weighing less.