Which of the following groups have seven valence electrons and are very reactive?

Column 1 alkali metals
Column 2 alkaline earth metals
Column 17 halogens
Column 18 noble gases

Column 17 halogens

Explanation:

To answer the question we need to know the following;

What are valence electrons?

What are halogens?

What is the reactivity of halogens?

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%