Question

The molecular mass of a salt of oxy acid of chlorine of a divalent metal which contains more number of oxygen atoms than its corresponding ‘-ic’ acid is 239 g/mole. What is the atomic mass of the metal?
[A] 24
[B] 39
[C] 40
[D] 30

Answer 1

We have to write down the oxy acids of chlorine and then find out the ones ending with ‘-ic’. Once we know that we can write the formula of the compound and find out the molecular mass in terms of a constant (which will be the atomic mass of the metal) and then equating it with the given molecular mass will give us the atomic mass of the metal.

Complete step by step answer:

As we know, chlorine has four oxy-acids: $HClO$ or hypochlorous acid, $HCl{{O}_{2}}$ or chlorous acid, $HCl{{O}_{3}}$ or chloric acid and, $HCl{{O}_{4}}$ or perchloric acid.

As we can see in the question the corresponding ‘-ic’ acid is mentioned therefore, the oxyacid is either chloric acid or perchloric acid.

As the metal given is divalent i.e. the metal can combine with two hydrogen atoms. Therefore, the formula of the oxy-acid of chlorine with the divalent metal will be $M{{(Cl{{O}_{4}})}_{2}}$; where, M is the metal.

Let us consider that the atomic mass of the metal is ‘x’.
Atomic number of Chlorine =17 ,
Atomic mass of Chlorine = 35.5,
Atomic mass of oxygen = 16.
Therefore, for four oxygen atoms, atomic mass = $16\times 4=64$.
Therefore, molecular mass of the compound = $x+2[35.5+64]$.

But, the molecular mass of the compound in the question is given as 239.
So, we can equate the given molecular mass with the calculated molar mass to solve for ‘x’:
$x+2[35.5+64]$ = 239
Or, x + 199 = 239
Or, x = 239-199 = 40
Therefore, the atomic mass of the metal is 40.
Thus, we can also say that the metal is calcium.

Therefore, the correct answer is option [C] 40.

Note:
It is important here to remember the oxy acids of chlorine to answer this question. It is also important to remember the atomic masses of the elements for calculation.
Here, we are not taking $HCl{{O}_{3}}$ for the calculation because the question says the molecular mass is given for the salt of oxy acid of chlorine which contains more oxygen atoms than its ‘-ic’ form. Among the two ‘-ic’ oxy acids, perchloric acid contains a higher number of oxygen atoms which means the molecular mass is given for the perchloric acid.