Atom economy

Quantitative chemistry

GCSE Chemistry (8462) · 4.3.3.2 · Foundation & Higher

ORIGINAL

Relative formula mass of the desired product (balancing number included) ÷ total relative formula mass of the reactants × 100 (chemistry only).

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Question

Ethanol can be made by fermenting glucose:

\mathrm{C_6H_{12}O_6(aq) \rightarrow 2C_2H_5OH(aq) + 2CO_2(g)}

. Calculate the percentage atom economy for making ethanol.

A_r

: C = 12, H = 1, O = 16. Give your answer to 3 significant figures.

Every step worked, with the reasoning.

1
$M_r(\mathrm{C_6H_{12}O_6}) = 6(12) + 12(1) + 6(16) = 180$
The total $M_r$ of all reactants — here there is only one.
2
$M_r(\mathrm{C_2H_5OH}) = 2(12) + 6(1) + 16 = 46$
Ethanol has six H atoms in total: five in C₂H₅ and one in OH.
3
$\text{desired product} = 2 \times 46 = 92$
Use the balancing number: one glucose makes two ethanol. Common error: using 46 instead of 92, which gives 25.6%.
4
$\text{atom economy} = \dfrac{92}{180} \times 100 = 51.1\%$
Check the masses balance: 92 + 2(44) = 180, so mass is conserved.

Answer: $51.1\%$ (3 s.f.)