Other Calculations
As well as being able to calculate the concentration of a solution, you must be able to do the following calculations. You should be able to solve these calculations just by using the definitions of gram equivalents, equivalence etc covered.
1 How much of a solution is needed to supply a certain amount of a component?
2 How much of a component is needed to prepare a solution of specified concentration?
How many millilitres of a solution of Sodium Chloride, concentration 0.230 N are needed to provide 3 grams of Sodium Chloride?
Find the number of equivalents of Sodium Chloride needed.
The gram equivalent of NaCl is RMM/1 = 58.5/1 = 58.5 g
Equivalents needed = Mass of NaCl Needed = 3
NaCl Gram Equivalent 58.5
Equivalents needed = 0.051282 Eq
The gram equivalent of NaCl is RMM/1 = 58.5/1 = 58.5 g
Equivalents needed = Mass of NaCl Needed = 3
NaCl Gram Equivalent 58.5
Equivalents needed = 0.051282 Eq
Find the volume of solution needed.
A Normality of 0.230 means that there are:
0.230 Equivalents of NaCl in 1000 ml of solution. Thus there are:
0.051282 Equivalents of NaCl in 0.051282/0.230 x 1000 ml of solution.
There are:
0.051282 Equivalents of NaCl in 222.9 ml of solution.
222.9 ml of solution will provide the required amount of Sodium Chloride.
A Normality of 0.230 means that there are:
0.230 Equivalents of NaCl in 1000 ml of solution. Thus there are:
0.051282 Equivalents of NaCl in 0.051282/0.230 x 1000 ml of solution.
There are:
0.051282 Equivalents of NaCl in 222.9 ml of solution.
222.9 ml of solution will provide the required amount of Sodium Chloride.
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You need to prepare 400 ml of a 4 mEq/litre solution of Potassium ions. How many milligrams (to the nearest milligram) of Potassium Chloride do you need?
Find how many grams of Potassium are needed.
A concentration of 4 mEq/litre means that there are 4 mEq of Potassium per 1000 ml. 400 ml of the solution must therefore contain:
400/1000 x 4 mEq = 1.6 mEq.
1 gram equivalent of Potassium weighs 39.1 g.
Thus 1 milliequivalent weighs 0.0391 g.
Mass of Potassium needed = No. of mEqs x Weight of 1 mEq = 1.6 x 0.0391 g
Mass of Potassium needed = 0.06256 grams
A concentration of 4 mEq/litre means that there are 4 mEq of Potassium per 1000 ml. 400 ml of the solution must therefore contain:
400/1000 x 4 mEq = 1.6 mEq.
1 gram equivalent of Potassium weighs 39.1 g.
Thus 1 milliequivalent weighs 0.0391 g.
Mass of Potassium needed = No. of mEqs x Weight of 1 mEq = 1.6 x 0.0391 g
Mass of Potassium needed = 0.06256 grams
Find the number of moles of Potassium needed.
Moles of Potassium = Mass/RMM = 0.06256 / 39.1 = 0.0016 moles
Moles of Potassium = Mass/RMM = 0.06256 / 39.1 = 0.0016 moles
Find the mass of Potassium Chloride needed.
1 mole of Potassium is provided by 1 mole of Potassium Chloride. Thus:
0.0016 moles of Potassium are provided by 0.0016 moles of KCl.
Mass of KCl needed = Moles x RMM = 0.0016 x 74.6 g = 0.1193 g
Mass of KCl needed = 119 milligrams to the nearest milligram.
1 mole of Potassium is provided by 1 mole of Potassium Chloride. Thus:
0.0016 moles of Potassium are provided by 0.0016 moles of KCl.
Mass of KCl needed = Moles x RMM = 0.0016 x 74.6 g = 0.1193 g
Mass of KCl needed = 119 milligrams to the nearest milligram.
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