Calculate the thermal capacity of 40 g of water. [Specific heat capacity of water = 4200 JKg-1°C-1]

Given: mass of water m = 40 g = 0.040 kg

specific heat capacity c = 4200 J kg⁻¹ °C⁻¹

Thermal capacity, C = m × c = 0.040 × 4200 = 168 J °C⁻¹

Thermal capacity of 40 g of water = 168 J °C⁻¹

The specific heat capacity is given per KILOGRAM, so convert 40 g to 0.040 kg first. Using 40 straight away gives 168 000, a thousand times too large — always check the unit printed beside the constant.

Keep the two ideas apart. SPECIFIC heat capacity belongs to the substance (water is 4200 J kg⁻¹ °C⁻¹ whatever amount you have). THERMAL capacity belongs to this particular object and depends on how much of it there is, which is why it is mass × specific heat capacity and why its unit has no mass in it — just J °C⁻¹.