Question

Percentage purity of a sample of gold is $85\%$. How many atoms of gold are present in its $1$ gram sample? (Atomic mass of gold $=197$u).

• A. $3.0\times {10}^{21}$
• B. $2.6\times {10}^{23}$
• C. $2.6\times {10}^{21}$
• D. $4.5\times {10}^{26}$

Answer 1

The correct option is C $2.6\times {10}^{21}$

Given,

$85$% pure gold sample

That means in $100gm$ sample $85gm$ gold is present

We are given with $1gm$ sample

$\therefore$ Mass of gold=$85\times {10}^{-2}$

We know, $1$ mole of gold=$197gm$

$197gm$ gold=$6.023\times {10}^{23}$ atoms of gold

$\therefore 85\times {10}^{-2}$ gm gold contains $\frac{85\times {10}^{-2}\times 6.023\times {10}^{23}}{197}$ atoms

$=2.598\times {10}^{21}$ atoms

$=2.6\times {10}^{21}$ atoms