MHT CET · Chemistry · Solid State

A metallic element crystallises in simple cubic lattice. If edge legth of the unit cell is \(3 A^{\circ}\), with density \(8 \mathrm{~g} / \mathrm{cc}\), what is the number of unit cells in \(100 \mathrm{~g}\) of the metal? \((\) Molar mass of metal \(=108 \mathrm{~g} / \mathrm{mol}\) )

A \(1.33 \times 10^{20}\)
B \(2 \times 10^{24}\)
C \(2.7 \times 10^{22}\)
D \(5 \times 10^{23}\)

Verified Solution

Answer & Solution

Correct Answer

(D) \(5 \times 10^{23}\)

Step-by-step Solution

Detailed explanation

\((\mathrm{B})\)
\(\mathrm{a}=3 Å=3 \times 10^{-8} \mathrm{~cm}\)
Volume of the unit cell \(\left(\mathrm{a}^{3}\right)=\left(3 \times 10^{-8}\right)^{3} \mathrm{~cm}^{3}=27 \times 10^{-24} \mathrm{~cm}^{3}\)
Mass of unit cell \(=27 \times 10^{-24} \mathrm{~cm}^{3} \times 8 \mathrm{~g} \mathrm{~cm}^{-3}\)
\(=216 \times 10^{-24} \mathrm{~g}\)
\(\begin{aligned} 216 \times 10^{-24} \mathrm{~g}=1 \text { unit cell}\end{aligned}\)
\(\therefore 108 \mathrm{~g}=\frac{108}{216 \times 10^{-24}}=5 \times 10^{23} \text {unit cells.} \)

Apply the relevant identity from the chapter and substitute the values established in the previous step, keeping the original constraint in view.

Use the simplified expression to isolate the unknown, then plug the result back into the question setup to confirm the working is internally consistent.

Cross-check against a boundary or limiting case. Both the dimensional balance and the qualitative behaviour at the extreme should agree with the candidate answer.

Combine the steps above to identify the correct option. The remaining choices each fail at least one of the consistency, dimensional, or boundary checks.

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