C4:Chemical Calculations

Question 1

How do you work out relative formula mass?

Answer 1

-Look up the relative formula mass in the periodic table,the atomic mass number
e.g Mg=24 or Cl=35.5
Then add them together
If there is a 2 next to it then multiply it by 2
e.g MgCl2 = 24 + (35.5 x 2) = 94

Question 2

Calculate the relative formula mass (Mr2) of H20…

Answer 2

Hydrogen = 1 x 2 = 2
Oxygen = 16

So 16+2 = 18

Question 3

Calcualte relative formula mass of H2SO4

Answer 3

Hydrogen = 1 x 2 = 2
Sulphur = 32
Oxygen = 16 x 4 = 64
Total (64+32+2) = 
= 98
Relative Atomic Mass = 98

Question 4

Calculate the percentage composition by mass of potassium in potassium hydroxide (KOH)

Answer 4

Formula = Ar x number of atoms of that element/Mr of the compound
x100
Atomic mass of potassium = 39 oxygen = 16 Hydrogen = 1
39 x 19
Relative atomic mass of KOH = K = 39 + O = 16 + H = 1 = 56
39 x 19/56 x 100 = 33.9%

Question 5

Whats the formula of percentage of mass in a compound?

Answer 5

Percentage of mass in a compound =

Atomic Mass x number of atoms of the element/Relative atomic mass of compound x 100

Question 6

If carbon has a relative atomic mass of 12 what does one mole equal?

Answer 6

-One mole of carbon would weigh 12g

Question 7

Calculate the number of moles in 90g of water(H20)…

Answer 7

Relative atomic mass = O = 16 H=1x2 = 2 =18
Moles = mass/relative atomic mass
=90/18=
Number of moles = 5

Question 8

Calculate the mass of 0.20 mol of potassium bromide (KBr)…

Answer 8

KBr = K = 39 +  Br = 80 = 119
Moles = 0.20 
Moles x Mr = Mass
0.20 x 119 =
23.8

Question 9

Show the mass is conserved : H2SO4(aq) + 2NaOH(aq) –> Na2SO4(aq) + 2H20(1)

Answer 9

H x 2 = 2 S = 32 Ox4 = 64 2Na=46 O=32 H=2
=178
Other side:
 Na2=46 S=32 O=64 H=4 O=32
=178
=conserved