# Using gas law to find moles of SO3 formed

• Chemistry
• physgirl
In summary, a chemist studying the reactions of pollutant SO2 placed a mixture of SO2 and O2 in a 2.0L container at 900K and a pressure of 1.95 atm. After the reaction occurred, gaseous SO3 formed and the pressure dropped to 1.65 atm. Using the ideal gas law, the final number of moles was found to be 0.0447 moles. However, since not all of the produced gas is SO3, the net loss of 1 mole of gas results in 2 moles of SO2 being produced. This means that the actual number of moles of SO3 formed is (0.053-0.0447)
physgirl

## Homework Statement

A chemist studying the rxns of pollutant SO2 places a mixture of SO2 and O2 in a 2.0L container at 900K and an P[init]=1.95 atm. When rxn occurs, gaseous SO3 forms and P falls to 1.65 atm. How many moles of SO3 formed?

## Homework Equations

PV=nRT but I'm not sure what else...

## The Attempt at a Solution

I was assuming both SO2 and O2 in the starting material are gases? So I used V=2L, P=1.95atm, and T=900K to find the number of moles of both gases combined, which turned out to be ~0.053 moles. And I'm not sure what to do with this number to get to the answer... I wrote out the balanced equation to be: 2SO2(g) + O2(g) --> 2SO3 (g) although I don't know if that matters... I can't assume that T stays the same, can I? the problem doesn't say anything about it...

 wait, can I start out by saying that... since you're not losing any gas, #moles of starting material (SO2+O2) has to be the same as #moles of final mixture (SO3+(leftover)SO2+O2)... and so use 0.053moles, given final pressure (1.65atm) and 2L (since it's a container it doesn't expand..?) to find the final temperature and go from there somehow?

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Try something called Chemical Equilibrium.

hmm...? :-/

physgirl said:
 wait, can I start out by saying that... since you're not losing any gas, #moles of starting material (SO2+O2) has to be the same as #moles of final mixture (SO3+(leftover)SO2+O2)... and so use 0.053moles, given final pressure (1.65atm) and 2L (since it's a container it doesn't expand..?) to find the final temperature and go from there somehow?
No, you have to assume that the volume of the container and the temperature are unchanged. And the number of moles is different from the number of atoms. It is the total number of atoms that is conserved, not the total number of molecules (or moles).

1. Find the final number of moles using the final pressure.
2. Translate the balanced equation into words.

Allright, don't try something called Chemical Equilibrium.
Try something called Follow What Gokul Pointed Out.

so I did P(i)/n(i)=P(f)/n(f)... and got the final #moles to be 0.0447moles of gas... but I'm not sure what you mean by "translate the balanced equation into words."... so from 3 moles of gas you make 2 moles of SO3 gas...?

Im guessing you can't assume that not all of 0.0447 moles of what's produced is SO3 because you can't assume that you had the exact amount of starting material needed to produce that, so the product gas mixture is a combination of all 3 gases?

Correct. All that's produced is NOT SO3.

so from 3 moles of gas you make 2 moles of SO3 gas
This is close. You need to expand on this idea. So, for every 3 moles consumed, there are 2 moles produced - resulting in a net loss of 1 mole.

Thus, 1 mole net loss => 2 moles SO2 produced.
So (0.053-0.0447) moles net loss => ?? moles of SO2 produced.

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## What is the gas law and how is it used to find moles of SO3 formed?

The gas law is a mathematical equation that relates the pressure, volume, temperature, and amount of gas in a system. To find the moles of SO3 formed, we can use the ideal gas law, which states that PV = nRT, where P is pressure, V is volume, n is the number of moles, R is the gas constant, and T is temperature.

## What information do I need to use the gas law to find moles of SO3 formed?

To use the gas law to find moles of SO3 formed, you will need to know the pressure, volume, and temperature of the system. These values can be measured or given to you in a problem.

## Can the gas law be used for any gas, or only specific gases like SO3?

The gas law can be used for any gas, as long as the units for pressure, volume, and temperature are consistent. In this case, we are using it to find the number of moles of SO3 formed, but the same equation can be used for any gas.

## How do I know which gas constant to use in the gas law equation?

The gas constant, R, has different values depending on the units used for pressure, volume, and temperature. Make sure to use the appropriate value for the units you are using. For example, if pressure is given in atmospheres and volume in liters, use the value 0.0821 L·atm/mol·K for R.

## What are some common mistakes to avoid when using the gas law to find moles of SO3 formed?

Some common mistakes to avoid include using the wrong units for pressure, volume, or temperature, using the wrong gas constant, and not converting between units if necessary. It is also important to make sure that all values are in the correct units before plugging them into the equation. Additionally, make sure to use the correct number of significant figures in your calculations to avoid rounding errors.

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