# Pressure and number of moles relationship questions

### What is the ideal gas law? (article) | Khan Academy

At a temperature of 20 °C and a pressure of 1 atmosphere, we find that it We can describe the relationship between the volume of a gas and the number of moles present using this How many moles are there in 48 dm3 of hydrogen gas? the relative atomic or formula masses to calculate these problems with gases. Learn how pressure, volume, temperature, and the amount of a gas are related to The simplicity of this relationship is a big reason why we typically treat gases as ideal, .. This shows that, as long as the number of moles (i.e. molecules) of a gas .. Alternatively, we could have solved this problems by using the molecular . Volume-temperature data for a 1-mole sample of methane gas at 1 atm are listed . The relationship between the volume and pressure of a given amount of gas at . how pressure affects a number of issues related to their comfort and safety.

To summarize, the total pressure exerted by a mixture of gases is the sum of the partial pressures of component gases.

This law was first discovered by John Dalton, the father of the atomic theory of matter. The total pressure of a mixture of gases is the sum of the partial pressures of the individual gases. Thus an ideal gas must be one whose properties are not affected by either the size of the particles or their intermolecular interactions because both will vary from one gas to another. A typical gas cylinder used for such depths contains What is the partial pressure of each gas at Use the ideal gas law to calculate the partial pressure of each gas.

Then add together the partial pressures to obtain the total pressure of the gaseous mixture. Calculate the partial pressure of each gas at The sum of the mole fractions of all the components present must equal 1. This conclusion is a direct result of the ideal gas law, which assumes that all gas particles behave ideally.

Consequently, the pressure of a gas in a mixture depends on only the percentage of particles in the mixture that are of that type, not their specific physical or chemical properties.

The value of the constant depends on the volume and the temperature.

### CH Lesson 1 - Gas Laws

The fact that P and n are directly proportional means not just that when n increases so does P, it means that when n is doubled, so is P; when n is tripled, so is P; when n is cut by one fourth, so is P. In general, when n increases or decreases, the ratio of the new to the old values of n will be the same as the ratio of the new to the old values of P. The value of the constant k determines how large P is compared to n, but no matter what value k has, large or small, the correspondence between the ratio of the change in n and the ratio of the change in P will exist.

What will happen to the pressure? Again, consider how the number of collisions and the force of the average collision will be affected. As before, since we have not changed the temperature, the force of the average collision will not change. Again, because the temperature has not changed, the collisions will be no more or less violent, but the number of collisions will change. In one third the volume, each gas particle has, on the average, one third the distance to go before hitting the wall of the container, and will therefore do so three times as often, increasing the pressure by a factor of three.

For an ideal gas, the pressure is inversely proportional to the volume of the container. Mathematically, we can express this by saying that the pressure is equal to a constant times one over the volume or that the pressure times the volume equals a constant.

This time, the value of the constant depends on the temperature and the number of moles. Inverse proportionality works the same way that direct proportionality does, except that when two quantities are inversely proportional, when one goes up, the other goes down. The correspondence between ratios remains.

When one goes up by a factor of two, the other goes down by a factor of two, and so on. Again, the constant determines the relative size of the two quantities. If the constant is close to 1, they will be close to the same size. If the constant is very large or very small, one will be much larger or much smaller than the other.

You can experience this relationship yourself in the lab. In the area labeled Exercise 9 you will find a syringe filled with a gas. The opening has been closed off so that you can change the volume of the syringe without changing the amount of gas present. When you start, the pressure inside the syringe is the same as the pressure outside the syringe. See what happens as you decrease and increase the volume occupied by the gas inside the syringe.

The pressure of the surrounding air does not seem like much. To get an idea of how strong it is, see how much work it takes to cut the volume of the syringe in half and hold it there.

If you cut the volume of the syringe in half, you will double the pressure inside and the additional pressure you will be working against will be the difference between the pressure inside the syringe 2 atm and outside the syringe 1 atm: If you increase the volume of the syringe by a factor of two, the pressure inside the syringe will drop to 0. What would happen to the pressure if you increased the temperature from K to K?

In this case, the number of gas particles and the volume that they occupy does not change. Since the temperature does change, so does their speed. How would this affect the force of the average collision of a gas particle with the wall of the container? Would it also affect the number of collisions that occur each second? Here, the collisions would become more violent and they would also become more frequent, in both cases because the gas particles would be moving more rapidly.

It turns out that, as long as the temperature is expressed in Kelvins, the pressure is proportional to the temperature and since the temperature increased by a factor of 1. It is not obvious why, if both the force of the collisions and their frequency increase, the increase in pressure would be proportional to the temperature. A detailed discussion is beyond us here, but it does make sense in a way if you consider that both the pressure and the temperature are sensitive to the same thing: When the temperature of a substance increases, it feels hotter to you because the molecules are hitting your skin both harder and more frequently, the same two factors that cause the pressure to increase.

Exercise 10 in the lab will give you an opportunity to verify this for yourself. First note the pressure on the gauge, then dip the metal globe into ice water, then boiling water and note how the pressure changes. Enter the temperature and pressure values into the first two lines of Exercise 10 in your workbook. The gauge attached to the top of the metal globe is rather heavy. If you let go while the globe is in the liquid, the whole set-up will tip over.

## Lesson 1: Gas Laws

It will take a few minutes for all the gas inside the globe to come to the temperature of the water bath. Be patient and wait for the pressure gauge to stop changing.

It will change rather slowly toward the end. The wall on the right is moveable — like a piston. It will stay put as long as the pressures inside and outside the container are the same. If the pressure inside increases, the wall will begin to move out and the resulting increase in volume will lower the pressure inside the container.

This will continue until the pressures inside and outside are once more equal.

**Mole Fraction and Partial Pressure Examples & Practice Problems**

The syringe you used earlier in this section is just such a container. If the plunger of the syringe is allowed to move freely, the pressure inside the syringe will always be exactly the same as the pressure outside. Any difference would cause the plunger to move in or out until the pressures were once again equal.

This, of course, assumes that there is no leakage past the seal between the moveable wall and the interior wall of the container. As you noted when you worked with the syringe, the seal in that case was pretty good.

Suppose that we take such a container and increase the number of moles of gas it contains from 1 mole to 2 moles. What will happen to the volume? This is similar to the example in which we added gas to a container at constant volume and found that the pressure increased.

Here, the volume will change in response to any change in pressure until the pressure returns to its original value. How must the volume change if we are to preserve the pressure unchanged? Increasing the number of moles of gas would cause the pressure inside the container to increase.

This would, in turn, move the piston out. In fact, the pressure will increase only enough to overcome the friction between the moveable wall and the interior walls of the container.

Still, we can predict the final state of affairs by imagining that first the pressure increases to its maximum, then the container expands to return the pressure to its original value. Since we double the number of moles of gas, the pressure would double. Then, to put the pressure back in half, the volume would have to double.