Solution Manual for Chemistry

A Molecular Approach with MasteringChemistry 2nd Edition by Tro ISBN 0321706153

9780321706157

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Experiment 6

Hydrates

For the Instructor

Concentrated nitric acid (~15 M) should be carefully diluted in 2.5 fold to give ~6 M

HNO3

Preparation Information - 24 students

Necessary Equipment – 24 students

Paper

Answers to Pre-Laboratory Questions

1. If a hydrate of the formula MCly xH2O decomposes when heated to produce HCl, what change would you expect to occur when a piece of blue litmus paper is held in the path of the vapor released? Explain.

The blue litmus paper should turn red as blue litmus paper turns red in the presence of acid and HCl is hydrochloric acid.

2. After 0.6523 g of CoCl2 6H2O is heated, the residue has a mass of 0.3423 g. Calculate the % H2O in the hydrate. What is the formula of the hydrate?

0.6523 g total – 0.3423 g CoCl2 = 0.3100 g H2O

0.3100 g H2O/0.6523 g total x 100% = 47.52 % H2O

0.3100 g H2O/18.02 gmol-1 = 0.01720 mol H2O

0.3423 g CoCl2/129.83 gmol-1 = 0.002637 mol CoCl2

0.01720 mol H2O/0.002637 mol CoCl2 = 6.523

Formula is approx. CoCl2 6.5 H2O

3. What safety precautions are necessary in this experiment?

Nitric acid is a strong acid. Be careful and wear gloves and goggles; lab coats are a goodsuggestion. Immediatelywashanyskinorclothingthatcomesincontact. Wash surfaces that come in contact so that someone else is not exposed accidentally. Have appropriate spill kit nearby.

4. CoCl2 is often used in hygrometers. Search the Internet to determine why. How does this relate to this experiment?

Hygrometer indicates relative humidity, CoCl2 is blue. As it picks up water, it forms CoCl2 6H2O which is pink. See for example http://wow.osu.edu/experiments/weather/humiditymonitor.html