   # Epsom salt is MgSO 4 · 7H 2 O. When heated to 70 to 80°C it loses some, but not all, of its water of hydration. Suppose you heat 2.465 g of Epsom salt to 75°C and find that the mass is now only 1.744 g. What is the formula of the slightly dehydrated salt? (a) Mg SO 4 · 6H 2 O (b) MgSO 4 · 5H 2 O (c) MgSO 4 · 4H 2 O (d) MgSO 4 · 3H 2 O ### Chemistry & Chemical Reactivity

9th Edition
John C. Kotz + 3 others
Publisher: Cengage Learning
ISBN: 9781133949640

#### Solutions

Chapter
Section ### Chemistry & Chemical Reactivity

9th Edition
John C. Kotz + 3 others
Publisher: Cengage Learning
ISBN: 9781133949640
Chapter 2.10, Problem 4RC
Textbook Problem
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## Epsom salt is MgSO4 · 7H2O. When heated to 70 to 80°C it loses some, but not all, of its water of hydration. Suppose you heat 2.465 g of Epsom salt to 75°C and find that the mass is now only 1.744 g. What is the formula of the slightly dehydrated salt? (a) Mg SO4 · 6H2O (b) MgSO4 · 5H2O (c) MgSO4 · 4H2O (d) MgSO4 · 3H2O

Interpretation Introduction

Interpretation: The formula of the slightly dehydrated Epsom salt is to be identified from the given options.

Concept introduction:

• Conversion formula for mass of a molecule and number moles,

Numberofmoles=MassingramsMolarmass

Molar mass of a molecule can be calculated from its molecular formula by taking the sum of atomic masses of all the elements present in it.

### Explanation of Solution

Given,

The mass of hydrated sample MgSO4.7H2O is 2.465g.

The mass of slightly dehydrated sample is 1.744g.

Therefore,

The mass of water driven off in the sample after heating is 2.465g1.744g=0.721g

Equation for finding number moles from mass of a molecule and molar mass is,

Numberofmoles=MassingramsMolarmass

Therefore,

The number of moles of water molecule lost is,

Numberofmoles=0.721g18g=0.04mol

Then, the number of moles of MgSO4.7H2O is,

Numberofmoles=2

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