Chapter 10, Problem 29PS

### Chemistry & Chemical Reactivity

10th Edition
John C. Kotz + 3 others
ISBN: 9781337399074

Chapter
Section

### Chemistry & Chemical Reactivity

10th Edition
John C. Kotz + 3 others
ISBN: 9781337399074
Textbook Problem

# A new boron hydride, BxHy, has been isolated. To find its molar mass, you measure the pressure of the gas in a known volume at a known temperature. The following experimental data are collected:Mass of gas = 12.5 mg Pressure of gas = 24.8 mm HgTemperature = 25 °C Volume of flask = 125 mLWhich formula corresponds to the calculated molar mass? (a) B2H6 (b) B4H10 (c) B5H9 (d) B6H10 (e) Bl0H14

Interpretation Introduction

Interpretation:

The molecular formula that corresponds to the given molar mass has to be calculated.

Concept Introduction:

density=PMRT

P-PressureM-MolarmassR-UniversalgasconstantT-Temperature

Explanation

Given that:

Â Â Â Â massâ€‰ofâ€‰theâ€‰compoundâ€‰=â€‰12.5â€‰â€‰mgâ€‰=â€‰0.0125â€‰gvolumeâ€‰â€‰â€‰â€‰â€‰=â€‰125â€‰mlâ€‰â€‰â€‰â€‰â€‰â€‰â€‰â€‰â€‰â€‰â€‰=â€‰0.125â€‰LPressureâ€‰=â€‰24.8â€‰mmâ€‰Hgâ€‰â€‰=â€‰0.0326â€‰atmTemperatureâ€‰=â€‰25Â°Câ€‰â€‰â€‰=â€‰298â€‰K

The molar mass and molecular formula of the given boron hydride Â Â Â Â can be calculated as

â€‚Â densityâ€‰=â€‰massvolumeâ€‰â€‰â€‰â€‰â€‰â€‰â€‰â€‰â€‰â€‰â€‰â€‰â€‰â€‰â€‰â€‰=â€‰0.0125â€‰g0.125â€‰Lâ€‰â€‰â€‰â€‰â€‰â€‰â€‰â€‰â€‰â€‰â€‰â€‰â€‰â€‰â€‰â€‰â€‰=â€‰0.1â€‰g/L

Using this density we can calculate the molar mass of the compound.

â€‚Â densityâ€‰=â€‰PMRT

â€‚Â Mâ€‰â€‰â€‰=â€‰â€‰dRTPâ€‰=â€‰0.1â€‰g/Lâ€‰Ã—â€‰0

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