Need help with pre ap chem!!

Question

Help!! I missed a whole week of school and didnt learn gas problems! Can you help me out with these practice questions??

Remember your pressure conversions: 1 atm = 14.7 psi = 760 mmHg = 101.3 kPa

- Remember any temperature in Celsius must 1st be converted into Kelvin by adding 273

- Remember that STP (standard temp and pressure) = 0 C or 273 K and 1 atm.

1. A balloon with a volume of 7.62 liters is containing 2.45 moles of gas. How many moles of gas remain if some gas is released and the balloon shrinks to a new volume of 5.25 liters (assume temp and pressure stay constant)

2. A 4.0 mole sample of gas occupies 4.4 liters at STP, which is 273 K and 1 atm. If more gas is added to the balloon, the volume increases to 6.7 liters and the number of moles is now 7.3 moles, and the pressure is now at 3.4 atm. What is the new temperature?

3. A sample of magic gas occupies 15.0 liters at 22 C and 754 mmHg. If the volume increases to 23.0 liters and the pressure drops to 723 mmHg, what’s the new temp?

4. A car tire has 65 moles of nitrogen gas (N2) and a volume of 87 liters. If the container has 776 more grams of N2 added, what will be the new volume of the tire? Hint: you need to determine convert grams into moles, and then add that to the original number of moles to get your 2nd number of moles.

5. A 12 liter tank contains 7 moles of oxygen. The pressure in the tank is 660 kPa at 25 C. If this oxygen tank were to leak, and lose 3 moles of O2 gas and the temperature now drops to 13 C, what would be the new pressure in the tank? Assume the volume of the tank stays the same at 12 liters.

Thanks!

Answer 1

The key to these problems is the manipulation of the Ideal Gas Law, typically written as PV = nRT.

In Problem 1, Temp and Pressure are constant, as is "R" the ideal Gas Constant. So, manipulating the Ideal Gas Law, we can make it:

The things on the right are constant. That means they will not change between time 1 and time 2. So, we can say:

or V₁/n₁ = V₂/n₂ or n₂ = V₂n₁/V₁ so n2 = 5.25•2.45/7.62 = 1.69 moles

The other problems are all similar in that you can manipulate PV = nRT to utilize it at time 1 and time 2 with certain elements constant. The critical thing is to ensure temperature is ALWAYS in Kelvin. The other elements need to be in matching units (example: if pressure is in atm at time 1, use atm at time 2)