[ibstudent]

Hi there!

Below are a couple of multiple choice questions I couldn't figure out about quantitative chemistry, I would really really appreciate it if someone could help me out...

1- Which expression represents the density of a gas sample of relative molar mass, Mr, at temperature T, and pressure, P?
A) (PMr)/(T)
B) (RT)/(PMr)
C) (PMr)/(RT)
D) (RMr)/(PT)

2- A 250cm^3 sample of an unknown gas has a mass of 1.42g at 35°C and 0.85 atmospheres. Which expression gives its molar mass, Mr? (R=82.05cm^3 atm k^-1 mol^-1)
A) (1.42*82.05*35)/(0.25*0.85)
B) (1.42*82.05*308)/(0.25*0.85)
C) (1.42*250*0.85)/(82.05*308)
D) (1.42*82.05*308)/(250*0.85)

Thanks in advance 

[Arkcon]

OK, lets start to work out the first one.  Do you know the combined gas law formula?  Do you know what the formula's terms are, and what each variable stands for?  Can you also relate the formula for density to parts of it?  And can you rearrange to match one of the possible answers?

This is how we help on this forum, we give you hints.  We don't dump completed answers.

[ibstudent]

OK, lets start to work out the first one.  Do you know the combined gas law formula?  Do you know what the formula's terms are, and what each variable stands for?  Can you also relate the formula for density to parts of it?  And can you rearrange to match one of the possible answers?

This is how we help on this forum, we give you hints.  We don't dump completed answers.

Thanks for replying

Okay lets see, to find the density and formula is mass/volume. Keeping this in mind the combined gas formula is pV/T = p1V1/T1. Mr is like mass, so should be in the numerator, that leaves me with A. C and D.

So I got a 33.33% chance of getting it right... hmm.... gut instinct says its D, I hope my gut is right xD.

Can you please tell me if the answer is D? I know I should do the working in this forum, but in this question I am a hopeless case (as seen above)

Thanks

[Borek]

Just checking units is enough.

What are units of RT/P?

[juanrga]

Hi there!

Below are a couple of multiple choice questions I couldn't figure out about quantitative chemistry, I would really really appreciate it if someone could help me out...

1- Which expression represents the density of a gas sample of relative molar mass, Mr, at temperature T, and pressure, P?
A) (PMr)/(T)
B) (RT)/(PMr)
C) (PMr)/(RT)
D) (RMr)/(PT)

2- A 250cm^3 sample of an unknown gas has a mass of 1.42g at 35°C and 0.85 atmospheres. Which expression gives its molar mass, Mr? (R=82.05cm^3 atm k^-1 mol^-1)
A) (1.42*82.05*35)/(0.25*0.85)
B) (1.42*82.05*308)/(0.25*0.85)
C) (1.42*250*0.85)/(82.05*308)
D) (1.42*82.05*308)/(250*0.85)

Thanks in advance 

Ok, you already showed that you know that density is M/V. To solve the questions by yourself you only need to know what is Mr and what equation relates PVT for gases {*}.

{*} Hint: consider ideal gas

[vmelkon]

density is mass/V as you stated.
Then we have the ideal gas formula = PV = nRT

n is the number of moles
How do you know how many moles you have? If you know the mass (g) and the molar mass (g/mol), then you can calculate your moles.

n = mass/M

then you get back to your PV = nRT equation ...