National Chemistry

Calculations using mole

ONE

If question tells you about mass of grams
or asks for mass or grams

TWO

If question tells you about concentration or mol/l
or asks for concentration or mol/l

Volume will also be mentioned.

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

ANSWERS

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
  154g

calculation   mass in question/answer
  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 ?

ANSWERS

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.

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