Question

Rashi mixed 10gm of iron with 5.8 gm of sulphur to form a compound. What will be the molecular mass of the compound produced?

• A. 98 gm
• B. 87 gm
• C. 78 gm
• D. 108 gm

Answer 1

The correct option is B

87 gm

Number of moles in 10g of iron = mass / molar mass

= $\left(\frac{10}{55}\right)$ = 0.181

Number of moles in 5.8g of sulphur = mass / molar mass

= $\left(\frac{5.8}{32}\right)$ = 0.181

Now, we will calculate the simplest ratio of the element by dividing the number of moles of each element by the smaller value of the two.
Since number of moles of each element is 0.181, therefore iron and sulphur are present in a ratio of 1 : 1.
Thus the empirical formula of the compound is FeS. Since FeS compound is known. So, the molecular mass is given as (55 + 32)g = 87