374 : 224 :: 100 : x
x = 59.89.
And the answer to the question is 59.89 per cent.
Again, suppose the question is of this kind:—"How much crystallised copper sulphate (CuSO4.5H2O) will be required to make 2 litres of a solution, 1 c.c. of which shall contain 0.0010 gram of copper?"
A litre is 1000 c.c., so, therefore, 2 litres of the solution must contain 0.001 gram × 2000, or 2 grams. How much crystallised copper sulphate will contain this amount of metal?
| Cu | = 63.3 | |
| S | = 32.0 | |
| O4 | = 64.0 | = 16×4 |
| 5H2O | = 90.0 | = 18×5 |
| ———— | ||
| 249.3 |
If 63.3 grams of copper are contained in 249.3 grams of sulphate, in how much is 2 grams contained.
63.3 : 249.3 :: 2 grams : x
x = 7.8769 grams.
The answer is, 7.8769 grams must be taken.
As a sample of another class of problem similar in nature to the last (but a little more complicated) take the following:—"What weight of permanganate of potash must be taken to make 2 litres of a solution, 100 c.c. of which shall be equivalent to 1 gram of iron?" In the first place the 2 litres must be equivalent to 20 grams of iron, for there are 20 × 100 c.c. in two litres. In the titration of iron by permanganate solution there are two reactions. First in dissolving the iron
Fe + H2SO4 = FeSO4 + H2
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