For a chemical reaction A → B, it was found that concentration of B is increased by 0.2 mol L⁻ in 30 min. The average rate of the reaction is ____________ × 10⁻¹ mol L⁻¹ h⁻¹. (in nearest integer)

Solution

Chemistry ma’am · 3 steps

Change the 30 minutes into hours first, then rate is change in concentration over time.

1. Rate from product. For A → B, the average rate is how fast [B] rises: Δ[B] divided by Δ t.
2. Match the units. The answer is asked per hour, so 30 min = 0.5 h.

   Rate = Δ[B]/(Δ t) = (0.2 mol L⁻¹)/(0.5 h) = 0.4 mol L⁻¹ h⁻¹

3. So. 0.4 = 4 × 10⁻¹ mol L⁻¹ h⁻¹, so the blank is 4.

The common slip: 0.4. Entering the rate itself, 0.4, ignores the × 10⁻¹ already in the blank. The number that goes in is 4.

The idea to keep. Always convert time into the unit the answer asks for before dividing, and read the power of ten in the blank carefully.

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