EK Gen Chem 1001 #187

[betterfuture]

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If the partial pressure of carbon dioxide is 30torr at STP, and nitrogen is the only other gas present, what is the percent by mass of carbon dioxide?

Approach
Pressure=760 torr
PpCO2=30 torr
PpN2=730 torr

percent by mass = mass of CO2/total mass of gas

The book explains to find mole fraction by taking 30/760 to get 0.004 by I don't get how 30torr=30 moles of gas? Could someone explain the steps and how to approach this problem. This is not clicking.

 

[betterfuture]

Okay so I found the percent mass composition by taking molar mass of CO2(44g/mol) and dividing by molar mass of CO2 and N2 (72 g/mol) to get 0.611, which is an answer choice but not the ANSWER to this question. The book explains the answer as so:

The mole fraction of the carbon dioxide is 30/760, or about 0.04. But since carbon dioxide has somewhat higher molar mass that nitrogen, it will have a somewhat higher percent by mass than mole fraction. That's enough to chose B. If you actually want to calculate it, suppose there are 100 moles of gas total. Then there's 4 moles of carbon dioxide and 96 moles of nitrogen. The total mass would be (4x44) + (96x28), and the fraction of carbon dioxide by mass would be (4x44)/ [(4x44) + (96x28)].​

First question, why are they figuring out the mole fraction? Second, when finding percent composition by mass of gases, are we not allowed to add the molar mass of each gas molecule, say, in this example, Co2 and N2? Is it because they are two separate molecules and not 1 molecule? The percent composition is applicable to 1 molecule not 2 different molecules, maybe? Help!

 

[aldol16]

The book explains to find mole fraction by taking 30/760 to get 0.004 by I don't get how 30torr=30 moles of gas? Could someone explain the steps and how to approach this problem. This is not clicking.

Partial pressure of a gas divided by total pressure is equal to its mole fraction in the gas mixture. That's Dalton's law of partial pressures.

Okay so I found the percent mass composition by taking molar mass of CO2(44g/mol) and dividing by molar mass of CO2 and N2 (72 g/mol) to get 0.611, which is an answer choice but not the ANSWER to this question. The book explains the answer as so:

What you found was the percent by mass composition of a mixture of 1 mole of N2 and 1 mole of CO2. Do your dimensional analysis and you will see.