[iced]

A 12.0g sample of a gas has a volume of 5.25 L at 30 C and 761 mm Hg. If 2.5 g of the same gas is added to this constant 5.25 L volume and the temperature raised to 64 C, what is the new gas pressure?

Not sure if my steps are correct but do I use PV=nRT to solve for  moles (n) then use a ratio knowing 12g of sample produce 0.2114213 mols and now I have 14.25 g sample and plug into equation PV=nRT?
If so how would I approach the ratio?

Thanks for the help

[Astrokel]

hey iced,

then use a ratio knowing 12g of sample produce 0.2114213 mols and now I have 14.25 g sample and plug into equation PV=nRT?

you just have to work out the mole ratio with the final mass and plug into the equation

n = mass/Mr

n/mass = constant, so you can work out the ratio

[Dan]

If 12 g gives you n = 0.211 mol, then how many moles does 14.5 g give you? Once you have the new n value you can solve for the pressure in the second case.

I would personally take the following approach to the problem, especially since the gas constant is not given in the question and I can't remember it for mmHg...

now,

PV = nRT, so let's say for the first case P₁V₁ = n₁RT₁

rearrange: (P₁V₁)/(n₁T₁) = R

Similarly, for the second case:

(P₂V₂)/(n₂T₂) = R

So, since both equal R...

(P₁V₁)/(n₁T₁) = (P₂V₂)/(n₂T₂)

Since V₁ = V₂, we simplify to...

P₁/(n₁T₁) = P₂/(n₂T₂) <- Equation (1)

Considering that n = m/m_r (m = mass; m_r = molecular mass

It follows that...

n₁ = m₁/m_r and n₂ = m₂/m_r

Substitute for n₁ and n₂ in equation (1)...

(P₁m_r)/(m₁T₁) = (P₂m_r)/(m₂T₂)

m_r's cancel out...

P₁/(m₁T₁) = P₂/(m₂T₂)

Rearrange:

P₂ = (P₁m₂T₂)/(m₁T₁)

All the values for the terms on the right hand side are given in the question, P₂ is the only unknown. Plug in the numbers and you have your answer.