   Chapter 2, Problem 163IL

Chapter
Section
Textbook Problem

Two general chemistry students working together in the lab weigh out 0.832 g of CaCl2 · 2 H2O into a crucible. After heating the sample for a short time and allowing the crucible to cool, the students determine that the sample has a mass of 0.739 g. They then do a quick calculation. On the basis of this calculation, what should they do next?(a) Congratulate themselves on a job well done.(b) Assume the bottle of CaCl2 · 2 H2O was mislabeled; it actually contained something different.(c) Heat the crucible again, and then reweigh it.

Interpretation Introduction

Interpretation: The procedure has to be done for the given experiment from the given data’s of CaCl2.2H2O has 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

Reason for correct answer:

Given,

The mass of hydrated sample CaCl2.2H2O is 0.832g.

The mass of sample after heating is 0.739g.

Therefore,

The mass of water molecules driven off in the sample is 0.832g0.739g=0.093g

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

Numberofmoles=MassingramsMolarmass

Therefore,

The number of moles of water molecule driven off from the sample is,

Numberofmoles=0.093g18g=0.00516mol

The number of moles of CaCl2.2H2O in sample is,

Numberofmoles=0.832g147

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