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Moles and solutions
By the end of section you should be able to…
•Calculate the amount of substance in mol, using solution volume and concentration
• Describe a solution’s concentration using the terms concentration and dilute.
The concentration of a solution is the amount of solute, in mol, dissolve per 1 dm3 (1000 cm3) of solution. The amount, in moles, in a solution is:
n = c x V(in dm3)
or
n = c x V(in cm3) 1000
Worked examples1. What is the amount in moles of sodium
hydroxide in 50 cm3 of a solution of concentration 2 mol/ dm3
Answern = c x V (in cm3)
1000
n = 2 x 50 1000 = 0.01 mol
2. If 0.4 mol of sodium carbonate is dissolved to make 200cm3 of solution, what is its concentration?AnswerRe-arrange the equation n = c x V(dm3)
c = n V(dm3) c = 0.4 mol 0.20 dm3
c = 2 mol/dm3
3. What volume of a solution of hydrochloric acid of concentration 0.5 mol/dm3 contains 0.15 mol?
Give the answer in cm3
AnswerRe-arrange the equation n = c x V(dm3)
V(dm3) = n / c
= 0.15 / 0.5
= 0.3 dm3 To convert 0.3dm3 to cm3, multiply 1000
0.3 x 1000 = 300 cm3
Standard solution
A standard solution is a solution of known concentration.
Standard solutions are normally used in titration to determine unknown
information about another substance.
Mass concentration( g/dm3)
•Find the amount, in mol,required for 1 dm3 of the solution . .
•convert moles to grams, i.e from mass = n x M .
Find the mass concentration, in g/dm3 for the following solutions(a) 0.02 moles of HCl dissolved in 500cm3 of solution. (b) 0.25 moles of HNO3 in 2 dm3
of solution.
Answers(a) n = mole x 1 dm3 V
n = 0.02 x 1 dm3 0.5 dm3
n = 0.04 mol/dm3
From mass = n x M
M(HCl) = 1 + 35.5 = 36.5
mass conc. (g/dm3) = 0.04 x 36.5
= 1.46 g/dm3
(b) 0.25 moles of HNO3 in 2 dm3 n = mole x 1 dm3
V
n = 0.25 mol x 1 dm3
2
n = 0.125 mol/dm3
from mass conc. = n x M
M(HNO3) = 1 + 14+ (16x3) = 63
i.e. mass conc. = 0.125 x 63
= 7.88 g/dm3
NOW TRY QUESTIONS 1 -3
PAGE 17
Dilute solution
• A dilute solution is a solution with a small amount of solute per dm3
• Normal bench solutions of acids usually have concentrations of 1 mol/dm3 or
2 mol/dm3. These are dilute solutions.
Preparing dilute solution
The simple formula usually used is as follows:
M1V1 = M2V2
Where
M1 = original concentration
V1 = original volume
M2 = new concentration
V2 = new volume
How can 500 cm3 of a 0.4 mol/dm3 solution of sulphuric acid be prepared from a solution of
concentration 2 mol/dm3.
Answer 2 x V = 500 x 0.4volume = 500 x 0.4
2 = 100 cm3
therefore, 100 cm3 of the original solution be further diluted by adding 400 cm3 of water to
make 500 cm3 of 0.4 mol/dm3
What is the concentration in mol/dm3 of 375 cm3 of dilute sodium hydroxide solution prepared by diluting 75 cm3 of sodium hydroxide solution of concentration 0.6
mol/dm3 with water?
Answer From M1V1 = M2V2
75 x 0.6 = 375 x M2
M2 = 75 x 0.6
375concentration, M2 = 0.12 mol/dm3