Question

Determine the mass and the number of molecules found in 2 mole of lithium hydroxide.

A. $23.95{\text{g }};{\text{ }}1.2044 \times {10^{24}}{\text{molecules}}$

B. $23.95{\text{g }};\;6.022 \times {10^{23}}{\text{molecules}}$

C. $48.0{\text{g }};\;6.022 \times {10^{22}}{\text{molecules}}$

D. $48.0{\text{g }};\;1.20 \times {10^{24}}{\text{molecules}}$

Answer 1

Hint :**** This question can be solved on the basis of mole concept and stoichiometry. As per definition, one mole of a substance is defined as $6.023 \times {10^{23}}$ particles (molecules, ions, atoms, etc.) of that substance. The molecular weight of a compound and atomic weight of an element contain one mole of molecules of that compound or one mole of atoms of that element.

Complete answer:

The molecular weight of lithium hydroxide = $\left[ {7 + 16 + 1} \right] = 24$g

So, as per definition, 1 mole of lithium hydroxide = 24 g

Therefore 2 mole of lithium hydroxide= $24 \times 2 = 48$g

Again one mole of molecules = $6.023 \times {10^{23}}$particles.

Therefore 2 mole of lithium hydroxide = $2 \times 6.023 \times {10^{23}}$ particles = $1.2 \times {10^{24}}$ molecules of lithium hydroxide.

So, the correct answer is option D.

Note: Number of moles of a substance whose weight is given can be calculated by the following formula, ${\text{n = }}\dfrac{{\text{w}}}{{\text{M}}}$, where w is the given weight of substance and M is its molecular weight.

The mass of one mole of a substance is called the Molar Mass of the substance. The molar mass in grams is equal to atomic/molecular/formula mass in atomic mass unit (u).

One mole of a substance, i.e. the amount $6.023 \times {10^{23}}$particles is called the “Avogadro’s Constant” or “Avogadro’s Number.”

The quantity “one mole” of a substance was introduced because the masses of atoms and molecules are so small that they cannot be measured directly.