# HELP - Molar mass changes at stp?

• rocketboy
In summary, the conversation is about calculating the molar mass of butane using data from an experiment and comparing the results at different temperatures and pressures. There is a discrepancy in the calculations and one of the participants realizes they made an error in the data. The expert summarizer notes that the molar mass should remain the same regardless of changes in temperature and pressure.
rocketboy
PLZ HELP - Molar mass changes at stp??

Hey, I have this lab report due tomorrow, and I seem to be getting some discrepencies in my calculations. I am calculating the molar mass of butane, without using the values from the periodic table, using data obtained by the experiment below:

Pressure: 101.09 kPa
Temperature: 23ºC --> 296 K
Initial Mass of Lighter: 18.19g
Secondary Mass of Lighter: 18.06g
Mass of Butane used: 0.15g
Volume of Butane: 50.2

I won't bother you with error propagation as it is easy to caculate, so disregard that. Below are my calculations for the molar mass of butane, at the temp and pressure given above and at STP (standard temp and pressure).
Am I doing this properly? Do my results make sense? (Ignore the "..." they are for placement)

Molar Mass of Butane at Given Temperature and Pressure:

50.2 +/- 1.0mL X 1.0L = 5.02 x 10^-2 L
......1000mL

P = nRT --> n = PV
...V.....RT

n = 101.09kPa (5.02 x 10-2L)
...8.31kPa.L/mol.K (296K)

n = 2.06 x 10^-3 molButane

M = m
...n

M = 0.15g .
...2.06 x 10^-3 molButane

M = 72.71g/molButane

Molar Mass of Butane at STP:

T1 = 296K P1 = PTotal – PH20 P2 = 101.3kPa
T2 = 273K = 101.09kPa – 2.81kPa V1 = 50.2mL
= 98.28kPa

50.2 mL X 273K X 101.3kPa = 47.7mL = 4.77 x 10^-2 L
...296K...98.28kPa

P = nRT --> n = PV
...V...RT

n = 101.3kPa(4.77 x 10^-2 L)
...8.31kPa.L/mol.K (273K)

n = 2.13 x 10^-3 molButane

M = m
...n

M = 0.15g .
...2.13 x 10^-3 molButane

M = 70.42g/molButane

I know I've done something wrong but don't know what.

Thanks you SO much,
-Jon

OMG! Sorry, i made the dumbest error. The mass of butane is 0.13g not 0.15...i subtracted the data wrong by 0.02 and it changed the answer by around 9g/mol!

but still, should i be getting different answers for each case? Why is this or is it not?

-jon

Last edited:
No, you should not be getting different answers. One mol of a quantity contains a specific number of molecules (Avogadro's number) and so has a specific mass. That doesn't change just because you change temperature and pressure (although the volume might.)

rocketboy said:
(snip)I won't bother you with error propagation as it is easy to caculate, (snip)
M = 72.71g/molButane (snip)

M = 70.42g/molButane

I know I've done something wrong but don't know what.

Thanks you SO much,
-Jon

You might try the "easy" part.

Oh I did the "easy" part in my lab notebook of course. Since i made the correction (post 2 of mine) i have values approx 61 and 64. But I think I'm using the wrong equation or something..because ivy is right, molar mass should always stay the same.

## 1. What is the definition of molar mass?

Molar mass is defined as the mass of one mole of a substance, usually expressed in grams per mole. It is a physical property that is unique to each element or compound.

## 2. How does molar mass change at STP?

At STP (standard temperature and pressure), the molar mass of a substance remains constant. This is because both temperature and pressure are defined constants at STP, so the molar mass is not affected.

## 3. Why is molar mass important in chemistry?

Molar mass is important in chemistry because it is used to convert between the mass of a substance and the number of moles of that substance. It is also used to determine the molecular formula of a compound.

## 4. How do you calculate molar mass?

To calculate the molar mass of a compound, you need to add up the atomic masses of all the atoms present in the compound. This can be found on the periodic table, and the unit for molar mass is grams per mole.

## 5. How does molar mass affect the properties of a substance?

Molar mass can affect the physical and chemical properties of a substance. For example, substances with a higher molar mass tend to have higher melting and boiling points, as well as greater intermolecular forces. It can also affect the reactivity and stability of a compound.

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