Question

A pure sample of cobalt chloride weighing 1.30 g was found to contain 0.59 g of cobalt and 0.71g of chloride on quantitative analysis. If the percentage composition of cobalt = x% and chloride = y%, then the value of x and y are, respectively
(a) 45.4,54.6
(b) 40.2,60.4
(c) 50.0,50.2
(d) None of the above

Answer 1

Hint: According to the law of definite composition which states that in a compound, the elements are present in a definite proportion by mass and the percentage purity of any substance/ material  can be calculated by dividing the mass of the each element by the total mass of the compound and then multiplying it by 100.

Complete step by step answer:
By quantitative analysis, we mean to evaluate the mathematical data and give the result in the form of numbers.
In pure samples of cobalt chloride, there are two elements i.e. cobalt and chloride which combine exactly in the ratio of elements by weight. This is known as the law of definite composition which states the given compound always contains exactly the same proportion of elements by weight.
The weight of cobalt chloride is 1.30 g (given) and the percentage of cobalt and chloride can be found dividing the mass of Co/Cl with the total mass of cobalt chloride and then multiplying it by 100.
1.30 g of the compound of cobalt chloride contains = 0.59 g of Co(given)
 1 g of the compound of cobalt chloride contains = $\frac {0.59} {1.30} $g of Co 100 g of the compound of cobalt chloride contains = $\frac {0.59\times 100} {1.30} $g of Co = 45.38%

Similarly, 1.30 g of the compound of cobalt chloride contains = 0.71g of Cl(given)
 1 g of the compound of cobalt chloride contains = $\frac {0.71} {1.30} $g of Cl 100 g of the compound of cobalt chloride contains = $\frac {0.71\times 100} {1.30} $g of Cl = 54.61%
Hence, the percentage composition of cobalt = 45.38% and chloride = 54.61%
Thus, option (a) is correct.

Note: Purity composition in any substance tells us about how pure the sample is and it can be indicated either quantitatively (i.e. to express something in terms of quantity) or qualitatively (which tells us about the qualities of the substance).