18. Which of the following reactions are endothermic or exothermic? Explain your answers. A. When a certain amount of potassium nitrate is dissolved in water, the water temperature drops from 23°C to 18°C. B. Jack cooks himself a steak. C. When cold water is mixed with sulphuric acid, the mixture can rapidly reach the boiling point of water. D. A flare burns up slowly.

The answer to your question is below

Explanation:

An endothermic reaction absorbs heat to proceed.

An exothermic reaction releases heat to proceed.

A. When a certain amount of potassium nitrate is dissolved in water, the water temperature drops from 23°C to 18°C. This is an example of an endothermic reaction because of the temperature diminishes.

B. Jack cooks himself a steak. This is an endothermic reaction because the steak does not cook itself it needs heat to be cooked.

C. When cold water is mixed with sulphuric acid, the mixture can rapidly reach the boiling point of water. This is an exothermic reaction because the temperature of water increases, which means that energy is been released.

D. A flare burns up slowly. I think it is an endothermic reaction because it was supply energy to proceed.

1. for the first question, we must find the mass of the anhydrous salt, mgso₄. the molar mass for mgso₄·7h₂o is 246.47 g/mol, while that of mgso₄ is 120.37 g/mol.7.834 g mgso₄·7h₂o * 1 mol mgso₄·7h₂o/246.47 g * 1 mol mgso₄/1 mol mgso₄·7h₂o * 20.37 g mgso₄/mol = 0.647 g mgso₄

2. mass of water = mass of sample - mass of anhydrous salemass of water = 7.834 - 0.647 = 7.19 gpercent water in hydrate = 7.19/7.834 * 100 = 91.74%