Question

An alloy has $Fe$, $Co$ and $Mo$ equal to 71%, 12% and 17% respectively. How many cobalt atoms are there in a cylinder of radius 2.50 cm and a length of 10.0 cm?

[The density of alloy is $8.20g/mL$. The atomic weight of cobalt = $58.9$.]

• A. $0.198\times {10}^{23}$
• B. $198\times {10}^{23}$
• C. $19.8\times {10}^{23}$
• D. $1.98\times {10}^{23}$

Answer 1

The correct option is C $19.8\times {10}^{23}$

$\vartheta =\pi r^{2}h=3.14\times (2-5{)}^2\times 10=196.25c{m}^3$

mass $=\left(8.2\right)\left(196.25\right)=169.25gm$

mass of Co $=\frac{12}{100}\times 1609.25=193.11g$

moles $=3.28\left(193.11/58.9\right)$

No. of atoms $=3.28\times NO$

$=19.75\times {10}^{23}atoms$