# Pressure and number of moles relationship quizzes

### Ideal Gas Law Formula

Self-assessment Quizzes on the molar gas volume and Avogadro's Law calculations Some handy relationships for substance Z below: moles Z = mass of . So the actual number of moles at this higher pressure will be 3 x = mol. Q. What type of relationship to pressure and volume have? answer choices. direct. inverse temperature & moles (amount of gas). volume & moles (amount of. Start studying Gas Law Quiz. Boyle's Law. The law that demonstrates the indirect relationship between pressure and volume. This law shows STP Numbers.

And there's basically two ways to change the internal energy.

If you want to add internal energy, i. That's one way to do it, to add heat. The other way to do it is to do work on the gas.

## Ideal Gas Law Formula

I could take this piston and push it down, and if you push this down hard enough it will squash this gas together, and those impacts with this piston while it's moving down will cause them to start moving faster and faster, that will also add internal energy to the gas. So if we wanted to write down a formula that told us how you could get a change in the internal energy, if I want to change the internal energy, Delta U, which is really just saying changing the kinetic energy, well there's two ways to do it.

I can add heat. If I added ten joules of heat I'd add ten joules to the internal energy, but I've also got to take into account this work being done, and so I can do plus the work done on the gas, and that's it, this is actually the first law of thermodynamics, it's the law of conservation of energy it says there's only two ways to add energy, internal energy to a gas. Let me talk a little bit more about this work done though cause getting the sign right is important.

If you're doing work on the gas compressing it you're adding energy to the gas, but if you let the gas push up on the piston and this gas expands pushing the piston up, then the gas is doing the work, that's energy leaving the system. So if the gas does work you have to subtract work done by the gas. If the outside force does work on the gas you add that to the internal energy. So you got to pay attention to which way the energy is flowing.

Work done on gas, energy goes in. Work done by the gas, energy goes out, and you'd have to subtract that over here. Let's say the gas did expand. Let's say the gas in here was under so much pressure that the force it exerted on this piston was enough to push that piston upward by a certain amount.

So let's say it started right here and it went up to here so that piston went from here to there no gasses escaping cause this is tightly fitting, but the gas was able to push it up a certain distance d, how much work was done?

You know the definition of work.

Work is defined to be the force times the distance through which that force was applied. So the work that the gas did was F times d, but we want this to be in terms of thermal quantities like pressure, and volume, and temperature. So what could we do? We can say that this volume, not only did the piston raise up, but there was an extra volume generated within here that I'm going to call Delta V, and I know that this Delta V has got to equal the area of the piston times the distance through which that piston moved because this height times that area gives me this volume right in here.

Why am I doing this? Cause look I can write d as equal to Delta V over A, and I can take this, I can substitute this formula for d into here, and something magical happens, I'll get work equals F times Delta V over A, but look F over A, we know what F over A is, that's pressure so I get that the work done by the gas is the pressure times Delta V.

This is an equation that I like because it's in terms of thermodynamic quantities that we're already dealing with. So work done you can figure out by taking P times Delta V but strictly speaking this is only true if this pressure remained constant, right?

If the pressure was changing, then what am I supposed to plug in here, the initial pressure, the final pressure? If the pressure's staying constant this gives you an exact way to find the work done. You might object and say wait, how is it possible for a gas to expand and remain at the same pressure? Well, you basically have to heat it up while the gas expands, that allows the pressure to remain constant as the gas expands.

- Properties of Gases

And now we're finally ready to talk about heat capacities. So let's get rid of this, and heat capacity is defined to be, imagine you had a certain amount of heat being added.

So a certain amount of heat gets added to your gas.

## Heat capacity at constant volume and pressure

How much does the temperature increase? That's what the heat capacity tells you. So capital C is heat capacity and it's defined to be the amount of heat that you've added to the gas, divided by the amount of change in the temperature of that gas. And actually, something you'll hear about often is the molar heat capacity, which is actually divided by an extra n here. Pretty simple but think about it. If we had a piston in here, are we going to allow that piston to move while we add the heat, or are we not going to allow the piston to move?

There's different ways that this can happen, and because of that there's different heat capacities. If we don't allow this piston to move, if we weld this thing shut so it can't move we've got heat capacity at constant volume, and if we do allow this piston to move freely while we add the heat so that the pressure inside of here remains constant, we'd have the heat capacity at constant pressure.

You still use the molar volume itself, but you think of it as the volume occupied by the formula mass of the gas in g and never think about moles! Methods of measuring how much gas is formed volume can be compared with theoretical prediction! You must make sure too much gas isn't produced and too fast!

A gas syringe is more accurate than collecting the gas in an inverted measuring cylinder under water shown below, but its still only accurate to the nearest cm3. You can collect any gas by this method. You can get a more accurate result by using an inverted burette instead of a measuring cylinder.

However, this method is no good if the gas is soluble in water!

### Heat capacity at constant volume and pressure (video) | Khan Academy

Burettes are calibrated in 0. In both methods the reaction is carried out in conical flask fitted with a sealing rubber bung, but a tube enabling the gas evolved to be collected in some suitable container. You need to put a cotton wool plug in the neck of the conical flask in case you lose any of the solution in a spray as the gas bubbles up - effervescence can produce an aerosol.

This method can be used for any reaction that produces a gas, but the gas is released into the laboratory, ok if its harmless. So the actual number of moles at this higher pressure will be 3 x 0. CO2 equation ratio is 1: Calculate its molecular mass. What volume of hydrogen is formed when How much magnesium is needed to make cm3 of hydrogen gas?