An average person needs about 10000 kJ energy per day. The amount of glucose (molar mass = 180.0 g mol⁻¹) needed to meet this energy requirement is ____________ g.
(Use : Δ C H(glucose) = -2700 kJ mol⁻¹)

Solution

Chemistry ma’am · 3 steps

Energy needed divided by energy per mole gives moles of glucose; then multiply by the molar mass.

1. Energy per mole. Δ_c H = -2700 kJ mol⁻¹ means burning 1 mol of glucose releases 2700 kJ. The minus sign only says heat is given out, so use 2700 when counting the energy supplied.
2. Moles needed. To supply 10000 kJ you need n = 10000/2700 = 3.704 mol of glucose.

   m = (10000 kJ)/(2700 kJ mol⁻¹) × 180.0 g mol⁻¹ = 666.7 g

3. So. About 666.7 g of glucose is needed each day, which rounds to 667.

The common slip: 48.6. Turning the ratio upside down, 2700/10000 × 180, gives 48.6, not 667. Energy needed goes on top, energy per mole goes below.

The idea to keep. For any fuel or food question, find moles as energy needed divided by |Δ_c H|, then convert moles to grams. Check that the units cancel to grams before you divide.

Answer from the NTA final answer key. Solution by PrepWiser.