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


if answer is a mass..

Mass of 1 mole


gram formula mass

number of mole

number of mole


volume in litre


cm3  divided by 1000 = litres


ml divided by 1000 = litres



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 in question/answer

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

                                                                               4 x Cl at 35.5 = 142                                                                                                                                                    



calculation        mass in question/answer

                                   mass 1 mole    





                                 =     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



   calculation         concentration   =   number of mole



                                                        =  0.25         ans  =  0.025 mol/l



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


calculation                volume in litres   =    number of mole



                                                               =   0.2         ans = 0.4







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.