Calculating concentration of a solution
-
The concentration is the amount of solute dissolved in a solvent to make
1 dm3.
When 1 mole of a compound is dissolved to make 1dm3 of solution the
concentration is 1 mol.dm-3
Formula:
c = n/V
c = concentration (mol.dm-3)
n = number of moles (mol)
V = volume (dm3)
n
(mol)
V
(dm3)
c
(mol.dm-3)
Worked Example
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Molar ration 2 : 1
4.92 g of hydrated magnesium sulphate crystals (MgSO4.nH2O) gave 2.40 g of anhydrous
magnesium sulphate on heating to constant mass. Work out the formula mass of the hydrated
magnesium sulphate and so the value of n.
MgSO4.nH2O MgSO4 + n H2O
Mr (MgSO4) = 24 + 32 + (4 x 16) = 120
n (MgSO4) = m/Mr = 2.4/120 = 0.02
Molar ratio = MgSO4.nH2O :MgSO4
1:1
0.02 : 0.02
Moles of MgSO4.nH2O = 0.02 moles
Mr (MgSO4.nH2O) = m/n = 4.92/0.02 = 246g
Total Mr of H2O = total Mr – Mr of MgSO4 = 246 – 120 = 126g
n (H2O) = m/Mr = 126/18 = 7
In an experiment to find the value of x in the compound
MgBr2.xH2O, 7.30 g of the compound on heating to
constant
mass gave 4.60 g of the anhydrous salt MgBr2. Find the
value of x. MgBr2.xH2O MgBr2 + x H2O