Changing molar mass at diff temp and pressures - help

• rocketboy
In summary, the conversation discusses the relationship between volume, pressure, mass, and molar mass of hydrogen gas at standard temperature and pressure. It is concluded that the molar mass of a substance is constant under these conditions, unless other factors such as radiation or nuclear reactions are taken into account. The conversation also addresses the importance of understanding this concept for a lab report evaluation.
rocketboy
Hi,

If I have a 50mL volume of hydrogen gas at standard temperature and pressure, and then increae the pressure, the volume is going to drop. Let's say that I know there is 1.5 grams of hydrogen gas in there. So if the volume drops, the mass of the hydrogen gas stays the same, but the number of moles of hydrogen will change, meaning that the molar mass is going to change. Is this correct? If not where did I go wrong and why?

If I am correct than that means molar mass of a substance is dependant on its surroundings...so are the molar masses in the periodic table at STP?

It's really important I know this for the evaluation of my lab report.
Thanks everybody,
-Jon

You couldn't be any wronger.The molar mass is the mass of 1 mole of substance = the mass of $N_{A}\simeq 6.023\cdot 10^{23} atoms/molecules$.This mass (essentially the mass of the atoms/molecules) is constant with temperature & pressure,if u neglect

a) Einstein's formula $E=mc^{2}$
b) Radiation loss /gain of atoms.
c) Nuclear reactions...

Daniel.

lol, that makes complete sense now, thanks.

1. How does changing temperature affect the molar mass of a substance?

As temperature increases, the average kinetic energy of particles in a substance also increases. This can cause the substance to expand and decrease in density, leading to a decrease in molar mass. However, for gases, an increase in temperature can increase the average speed of particles, resulting in an increase in molar mass.

2. How does changing pressure affect the molar mass of a substance?

Pressure and molar mass are indirectly proportional. As pressure increases, the volume of a substance decreases, leading to an increase in density and molar mass. Conversely, a decrease in pressure leads to an increase in volume and a decrease in molar mass.

3. Are there any exceptions to the relationship between temperature/pressure and molar mass?

Yes, there are some exceptions. For example, some substances experience sublimation, where they change directly from a solid to a gas without becoming a liquid. In these cases, the molar mass can remain constant even with changes in temperature and pressure.

4. How can I calculate the molar mass at different temperatures and pressures?

To calculate the molar mass at different temperatures and pressures, you can use the ideal gas law equation: PV = nRT. This equation relates the pressure (P), volume (V), number of moles (n), gas constant (R), and temperature (T) of a gas. By rearranging this equation, you can solve for molar mass (M).

5. In what situations would changing molar mass at different temperatures and pressures be important to consider?

Changing molar mass at different temperatures and pressures is important to consider in various scientific fields, such as chemistry, physics, and engineering. It is especially relevant in the study of gases and their behavior, as well as in industrial processes that involve temperature and pressure changes, such as manufacturing and chemical reactions.

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