Amounts of substances (3.2) (M)

Question 1

What are chemical amounts measured in?

Answer 1

moles

Question 2

What is the unit for moles?

Answer 2

mol

Question 3

What is the mass of one mole of a substance in grams numerically equal to?

Answer 3

its relative formula mass

Question 4

One mole of a substance contains the (…) number of the stated particles, atoms, molecules or ions as one mole of any (…) substance.

Answer 4

same

other

i.e. 1 mol of (carbon) = 1 mol of (helium)

Question 5

What is the Avogadro constant?

Answer 5

the number of atoms, molecules or ions in a mole of a given substance

Question 6

What is the value of the Avogadro constant?

Answer 6

6.02 x 10²³ per mole

Question 7

What do the symbols >> and << mean?

Answer 7

much greater/less than

Question 8

In one mole of carbon (C) the number of atoms is the (…) as the number of molecules in one mole of carbon dioxide (CO2)

Answer 8

same

Question 9

how do you calculate the number of moles?

Answer 9

n = m (g) / Mr

You may need to multiply Mr or m if number in front of formula (i.e. number of moles) is not 1

Question 10

H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O

Calculate the mass of sodium sulfate made when 20 g of sodium hydroxide reacts with excess sulfuric acid. (A_r of H = 1, A_r of O = 16, A_r of Na = 23, A_r of S = 32)

Answer 10

moles of sodium hydroxide = 20g / 2 x (23 + 16 + 1) = 20/80 = 0.25mol

mass of sodium sulphate = 0.25mol x 142 = 35.5g

Question 11

In a chemical reaction, why is it common to use in excess of a reactant?

Answer 11

to ensure that all of the other reactant is used

Question 12

What is the limiting reactant?

Answer 12

the reactant that is completely used up (limits the amount of products)

Question 13

When calculating the amount of products, should we use the mass of the limiting reactant or the reactant in excess?

Answer 13

limiting reactant (as that will be used up first)

Question 14

A + B ⇢ C

3 mol of A + 5 mol of B ⇢ 3 mol of C

Which one of A or B is the limiting reactant?

Answer 14

A

Question 15

P + 2Q ⇢ R

4 mol of P + 6 mol of Q ⇢ 3 mol of R

Which one of P and Q is the limiting reactant?

Answer 15

Q (would need 8 mol to match P)

Question 16

What is the formula for concentration?

Answer 16

concentration = mass (or moles) / volume

Question 17

What are units of concentration (within a solution)?

Answer 17

g/dm³ or g dm^-3

mol/dm³ or mol dm^-3

M

Question 18

Calculate the concentration of a substance when the mass is 200g and the volume is 800m³

Answer 18

200 / 0.8 = 250g/dm³

Question 19

What will happen to the concentration if we increase the mass at the same volume?

Answer 19

The concentration will increase

Question 20

What will happen to the concentration of a substance if we increase the volume at the same mass?

Answer 20

The concentration wil decrease

Question 21

Determine the limiting reactant: N₂ + 3H₂ ⇌ 2NH₃

Nitrogen: 28g, Mr - 14

Hydrogen: 12g, Mr - 1

Answer 21

1 3 ⇌ 2

N: 28 / (2 x 14) = 1 mol

H: 12 / (3 x 1 x 2) = 2 mol

1 mol: 2 mol

So N is limting reactant

Question 22

One mole of a substance is the relative formula mass in (…)

Answer 22

grams

Question 23

Te relative atomic mass of an element compares the mass of an atom of an element with the mass of (…)

Answer 23

¹²C