SC9c Moles - Higher

Question 1

what is the number for the avogadro constant?

Answer 1

6.02 x 10²³

Question 2

6.02 x 10²³ is the same as

Answer 2

1 mole

Question 3

mass =

Answer 3

moles x mr

Question 4

number of particles =

Answer 4

avogadro constant x moles

Question 5

Calculate the number of molecules in 9 g of hydrogen, H2

Answer 5

Moles = Mass ÷ Mr
= 9g ÷ 2
= 4.5 moles
1 mole = 6.02 x 10²³ particles
so 6.02 x 10²³ x 4.5 = 2.71 x 10²⁴ molecules

Question 6

Calculate the mass of 1.806 × 1025 molecules of ammonia, NH3

Answer 6

moles = number of atoms ÷ Avogadro’s number
= 1.806x10²⁵ ÷ 6.02x10²³
= 30 moles
Mass = Mr x Mole
= 17 x 30
= 510 g

Question 7

2H2 + O2 → 2H2O how many moles of hydrogen and oxygen is reacting to form how many moles of water?

Answer 7

2 mols of hydrogen and 1mol oxygens is reacting to produce 2 mols of water

Question 8

what does excess mean?

Answer 8

more than is required for the reaction to go to completion

Question 9

What is the limiting reactant of the reaction?

Answer 9

The other reactant that will be all used up completely

Question 10

What does the limiting reactant determine?

Answer 10

The quantity of the product produced as the one in excess wont be fully used up

Question 11

Determining the limiting reactant
23g of Hydrogen and 178g oxygen are reacted together to water.
2H2 + O2 → 2H2O
Which is the limiting reactant?

Answer 11

Moles H2 = 23/2 = 11.5mol
Moles O2 = 178/32 = 5.5625mol
Looking at the stoichiometry 2 H2
: 1 O2
We have 11.5 moles H2
so we need 11.5/2 mol O2= 5.75mol.
We only have 5.5625 mol O2 so O2 is the limiting reactant

Question 12

1.5g of ammonium chloride and 4.00g of calcium hydroxide are heated together to form ammonia.
2NH4Cl + Ca(OH)2 → 2NH3 + CaCl2 + H2O
Which is the limiting reactant?

Answer 12

Moles Ca(OH)2 = 0.0541mol
Moles NH4Cl = 0.0280mol

Looking at the stoichiometry 2 NH4Cl : 1 Ca(OH)2
We have 0.0280 moles NH4Cl so we need 0.0280/2
moles Ca(OH)2 = 0.014.
We have 0.0541 moles Ca(OH)2 so Ca(OH)2 is not the
limiting reactant

Question 13

What does stoichiometry mean?

Answer 13

The molar ratio of the reactants
and products in a chemical reaction

Question 14

1.5g of ammonium chloride and 4.00g of calcium hydroxide are heated together to form ammonia.
2NH4Cl + Ca(OH)2 → 2NH3 + CaCl2 + H2O
Calculate the mass of ammonia formed

Answer 14

Moles NH4Cl = 0.0280mol
So moles NH3 = 0.0280mol
Mass = Moles x Mr
Mr NH3 = 14 + (3x1) = 17
Mass = 0.0280 x 17 = 0.476g

Question 15

5.24g of Sodium hydroxide reacted with Sulfuric acid to form Sodium sulfate.
2NaOH + H2SO4 → Na2SO4 + 2H2O
Calculate the mass of sodium sulfate formed.

Answer 15

Moles NaOH = 0.131mol
2 Mol NaOH : 1 Mol Na2SO4
So moles Na2SO4 = 0.131/2 = 0.0655mol
Mass = Moles x Mr
Mass = 0.0655 x 142 = 9.30 g

Question 16

10.8g of aluminium reacted with 42.6g of chlorine, Cl2, to produce aluminium chloride, AlCl3. Deduce the balanced equation for the reaction.

Answer 16

Moles 10.8/27 = 0.4 42.6/71 = 0.6
Divide by small 1 1.5
Whole num ration 2 : 3
So 2 mol of Al reacts with 3 mol of Cl2. The equation is completed by adding the formula of the product and balancing in the normal way.
2Al + 3Cl2 -> 2AlCl3

Question 17

Calculate the number of molecules in 90g of water, H2O. (2 mark)

Answer 17

Number of moles of water = 90/18 = 5 (1)
Number of molecules of water = 5 x (6.02x1023)
= 3.01x1023 (1)