Question

A cylinder contains 250 g of Helium at 200 K. The external pressure is constant at 1 atm. The temperature of the gas inside the cylinder is then lowered by 85 K. Calculate q for this system in response to the change made. Include units of J when entering your answer.

Heat capacity for Helium = 20.8 J/(mol K) Molecular weight for Helium = 4 g/mol

I tried doing the q=Cp*m*dT and got 110500 J but it was incorrect.

Answer 1

Heat change (q) = mass *specific heat * change in temperature

     q = Cp*m*dT

as in numerical heat capacity (Cp) is given in J/(Mol K) so we can use instead of mass (m) the symbol n ( number of moles of gas )

means in numerical given mass of helium gas is 250 g

Molecular weight of Helium gas is 4 g / Mol

n=m/M

n=250 g/4 g/mol

n=62.5 mol

change in temp(dT)=97 K (given)

heat capacity = 20.8 J/mol K (given)

q=Cp*n*dT

q=20.8 J /mol K *62.5 mol * 97 K

q=126100 joule