Question

1. Assuming one equivalent of NaBH4 can reduce 4 equivalents of carbonyl functional groups, calculate the minimum amount of NaBH4 required to fully reduce 0.35 mmol of Benzil. Indicate if the the NaBH4 or the carbonyl group containing starting material is the limiting reagent.

Answer 1

Answer: Amount of $NaBH_4$ required will be 66.2025 g

Explanation:

We are given:

1 equivalent of $NaBH_4$ can reduce 4 equivalents of carbonyl functional groups.

Benzil has 2 carbonyl functional groups. Thus, 1 equivalent of $NaBH_4$ will reduce 2 equivalents of benzil.

Moles of Benzil given = 0.35 mmol = $3.5\times 10^{-4}mol$    (Conversion factor: 1 mol = 1000 mmol)

By Stoichiometry:

2 moles of benzil reacts with 1 mole of $NaBH_4$

So, $3.5\times 10^{-4}mol$ of benzil will react with = $\frac{1}{2}\times 3.5\times 10^{-4}=1.75\times 10^{-4}mol$ of $NaBH_4$

To calculate the mass of $NaBH_4$, we use the equation:

$\text{Number of moles}=\frac{\text{Given mass}}{\text{Molar mass}}$

Moles of $NaBH_4=1.75\times 10^{-4}mol$

Molar mass of $NaBH_4$ = 37.83 g/mol

Putting values in above equation, we get:

$1.75\times 10^{-4}mol=\frac{\text{Mass of }NaBH_4}{37.83g/mol}\\\\\text{Mass of }NaBH_4=66.2025g$

Hence, amount of $NaBH_4$ required will be 66.2025 g