National Chemistry

Calculations using mole If question tells you about mass of grams

or asks for mass or grams

If question tells you about concentration or mol/l

or asks for concentration or mol/l

Volume will also be mentioned. mass given in question

or

Mass of 1 mole

or

gram formula mass

number of mole

number of mole

concentration..

volume in litre

cm3  divided by 1000 = litres

ml divided by 1000 = litres

ONE

TWO

1. Calculate the mass mole of SO2 .

2. Calculate the mass of 2.5 mole of NH3 ..

3. How many mole are there in 38.5g of CCl4

Questions 1 to 3 only use triangle 1. Mass or grams are mentioned. Concentration is not.

1.  Triangle not needed. Just calcualte mass of one mole from the formula

SO2  has  1 x S at 32 = 32

2 x O at 16 = 32

64   ans = 64g

2. Use triangle You have both bottom parts so answer will be a multiplication.

number of mole  x  mass of 1 mole

2.5 x  mass 1 mole NH3          1 x N at 14 = 14

3 x H at   1 =   3

=   17g

calculation       number of mole  x  mass of 1 mole

2.5              x        17

= 42.5g

3. You have top and bottom right of triangle so this will be a division.

mass 1 mole                        1 mole CCl4 is  1 x C at 12   =   12

4 x Cl at 35.5 = 142

154g

mass 1 mole

38.5

154

=     0.25 mole

4. Calculate the number of mole in a 50ml solution of 0.1 mol/l socium hydroxide.

5. Calculate the concentration of a  100ml of solution which contains 0.25 mole hydrochlric acid .

6. What would be the volume of  solutiion if 0.2 mole silver nitrate  was used to make a solution of concentration 0.5 mol/l ?  # If both triangles are required then this figure - number of moles -will be the same

Triangle TWO is used

Q4 has the unit of concentration - mol/l

Q5 has the word concentration

Q6 has the word concentration - calculating volume is seldom asked question

4. Asked about top of triangle; given both bottom parts  - therefore this is a multiplication.

calculation        mole = concentration x litres

=  0.1  x  50 ml (divide by 1000 to get litres)

=  0.1   x  0.05

=  0.005 mole

5. Given volume (from bottom of triange) and number of mole (from top) - therefore a division

change volume to litres      = 100          =  0.1 litre

1000

calculation         concentration   =   number of mole

litre

=  0.25         ans  =  0.025 mol/l

0.1

6. Given concentration (from bottom of triange) and number of mole (from top) - therefore a division

calculation                volume in litres   =    number of mole

concentration

=   0.2         ans = 0.4

0.5

NOTE

In questions not involving mass (or grams) such as questions 4 to 6 there is no need to know the formula of the chemical.

Formulae are only required if mass needs to be calculated.

For questions involving CONCENTRATION and MASS -that is, using both triangles click here.