You need to use the ideal gas law (PV=nRT) and solve for n. ((3.50atm•10.0L)/(0.0821(L•atm/mol•K)•304K) = n = 1.40 moles. 1 mole of Cl2 = 70.9 gm/mole. The mass would be 99.43 gm
The climate would become colder...
Density(D) is defined as Mass(M) divided by Volume(V).
The formula for Density is:
D = M / V.
Another way to remember the formula for Density is to remember "Mass per unit of volume".
I hope this helps!