Practice Problems Week 8

1. What is the volume of a gas sample at 755 mmHg if it occupies 6.75 L at 875 mmHg? (Assume constant T & n)

OK. First we decide what form of the
General Gas Law (GGL) or Ideal Gas Law (IGL) we use. We are not given a number
of moles, a density or a number of molecules, so we don’t use the IGL. It looks
like we have a pressure and volume and a second pressure. So, it’s a PV
problem. So, it’s P_{1}V_{1} = P_{2}V_{2} which
comes from Boyle’s Law. We’re given P_{1 =} 875 mmHg and V_{1} = 6.75 L. We could have chosen the subscripts 2 if we wanted to). We’re also
given P_{2}. The question asks for V_{2}. So let’s take Boyle’s
Law and solve for V_{2}:

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2. What
is the temperature in °C of 5.75 m^{3} of a gas if at 2.68 m^{3} it has a temperature of 27°C?

It looks like this
problem deals with temperature and volume. So we use the form of the gas law
with those variables: . The question asks
for the final temperature (T_{2}), so let’s solve for that and then
substitute all of the given stuff into it:

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But we ALWAYS
remember that we have to use temperature in KELVINS when we use gas equations.
So we gotta convert the temperatures first: T_{1} = 27ºC + 273.15 = 300K. Remember the rules for addition and subtraction
regarding decimal places. So now we can substitute in our equation:

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3. What is the volume of a sample of gas at 25°C if it occupies 24.4 L at 50°C? (Assume constant P & n)

This looks like another volume – temperature problem. But
this time we have to calculate a volume. So let’s take our general V-T
equation: and solve it for a
temperature. It looks like the final temperature, T_{2}, is 25ºC, so T_{1} = 50ºC and V_{1} is 24.4 L. So let’s solve for V_{2} and
substitute values:

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