A sample of gas contains 3 moles of argon and 5 moles of krypton. If the argon exerts a pressure of 210 torr, what is the pressure of the krypton? What is the total pressure?

Hello!
This is a situation where two different gases are mixed in a common volume V, and they possess the same temperature T.  It is also important that these gases don't react with each other. In our case these gases are noble and don't react with anything.
Dalton's law of partial pressures states that for such a situation the total pressure is the sum of the partial pressures of both gases, P = P_(Ar) + P_(Kr). From the ideal gas law we know that
P_(Ar) = n_(Ar)*(RT)/V  and  P_(Kr) = n_(Kr)*(RT)/V.
R is a constant, T and V are the same for both gases, n is a quantity of a corresponding gas in moles.
Therefore P_(Ar)/P_(Kr) = n_(Ar)/n_(Kr),  so
P_(Kr) = P_(Ar)*n_(Kr)/n_(Ar) = 210*5/3 = 350 (To rr),
and the total pressure is  210+350 = 560 (To rr).
The answer: the pressure of krypton is 350 Torr. The total pressure is 560 Torr.