mass & concnentration - Standard Grade Chemistry

National Chemistry

Calculations using mole

If question tells you about mass of grams

or asks for mass or grams

mass given in question

or

if answer is a mass..

number of mole

Mass of 1 mole

or

gram formula mass

number of mole

volume in litre

cm3 divided by 1000 = litres

ml divided by 1000 = litres

concentration..

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 of sodium hydroxide (NaOH) in a 50ml solution of 0.1 mol/l .

ANSWER

Both triangles are required - mol/l (unit of concentration is given) and mass is asked for.

The question should give two figures in one triangle and onlyone figure in the other. There is also a space in a triangle which will be the answer. The first job is to position these.

No point in second guessing which is which - just start the figures given.

i) 50ml is a volume - this goes in to triangle TWO at position F (50 divided by 1000 to put in litres = 0.05 litres)

ii) 0.1 mol/l is a concentration - this goes into trinagle TWO at position E

The third figure is harder to see - it is the chemical formula to get the mass of 1 mole for triangle ONE position B

iii) 1 mole of NaOH = (1 x Na) + (1 x O) + (1 x H)

= (1 x 23) + (1 x 16) + (1 x 1) = 40g

(iv) Where is the answer ? Calculate the mass so the answer is in triangle ONE at position A.

*Thus we have*:

*from triangle 1 from triangle 2*

ANSWER = missing figure space C concentration x volume = missing figure space D

mass 1 mole

ANSWER = number of mole 0.1 x 0.05 = number of mole

mass NaOH

* triangle TWO has the two figures so it is the first calcualtion*

ANSWER = number of mole 0.1 x 0.05 = number of mole = 0.005 mole

40

* the number of mole is the same for both triangles - shift the answer from triangle TWO over to triangle ONE*

ANSWER = 0.005 =number of mole

40

ANSWER = 0.005 x 40 = 0.2g

4.

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 ?

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