Question

A sample of potato starch was ground in a ball mill to give a starch-like molecule of lower molecular weight. The product analysed was 0.086 % phosphorus. If each molecule is assumed to contain one atom of phosphorus, what is the molecular mass of the material?

• A. $3.42\times {10}^4$ amu
• B. $3.72\times {10}^4$ amu
• C. $3.6\times {10}^4$ amu
• D. None of these

Answer 1

Answer: (C)

$3.6\times {10}^4$ amu

Since the percentage of $P$ is $0.086$

$\therefore$ $0.086g$ of $P$ will be present in $100g$ of starch.

Then, $1g$ of $P$ will be present in $\frac{100}{0.0866}$ g of starch

The atomic mass of $P$ is $31a.m.u$.

$31a.m.u$ of $P$ will be contained in $\frac{100\times 31}{0.0866}a.m.u$ of Starch.

$\therefore$ Molecular weight of material= $3.6\times {10}^{4}a.m.u$