Gas Stoichiometry: Videos & Practice Problems

Stoichiometry is the study of the quantitative relationships in a balanced chemical equation, and gas stoichiometry specifically deals with reactions that produce gases. To perform gas stoichiometric calculations, one must understand the ideal gas law:

$PV=nRT$

which allows us to determine the moles of a gas given its pressure (P), volume (V), and temperature (T). Using a stoichiometric chart, we can convert the known quantity of a reactant or product, such as the moles of a gas, to find the unknown quantity of another substance in the reaction. This process involves a "jump" from known to unknown quantities, requiring a mole-to-mole comparison based on the coefficients of the balanced equation. Mastery of stoichiometry is essential for predicting the amounts of reactants and products in chemical reactions involving gases.

Gas Stoichiometry Concepts

Now here we're going to talk about gas stoichiometry. But first, recall that stoichiometry deals with the numerical relationship between compounds in a balanced chemical equation. And when we say gas stoichiometry, that deals with stoichiometric calculations of chemical reactions that produce gases.

Now, when talking about gas stoichiometry, we have to employ our stoichiometric chart. Within this chart, we're going to use the given quantity of a compound, in this case possibly a gas, to determine the unknown quantity of another compound within our balanced chemical equation.

Now that we know what gas stoichiometry is and how it connects to an idea of stoichiometry, which we've seen before, click onto the next video and let's take a better look at this stoichiometric chart.

Gas Stoichiometry Chart

So here we're going to take a look at the stoichiometric chart. Here we have 4 moles of silver solid reacting with one mole of oxygen gas to produce 2 moles of silver oxide solid. Here they give us the volume, pressure, and temperature of our oxygen gas, and we're asked to determine the grams of our silver oxide.

Realize here that if we're thinking of the ideal gas law, PV=NRT. By giving us the pressure, volume and temperature, we can isolate the moles of that particular gas. O here remember moles=pressurevolumeRT. So we would say that giving us pressure volume over RT since it's given to us that the amount of given that will directly feed into moles of given or they could give us the grams of one of the other compounds or elements within a balanced reaction.

And so we'd go from grams of given still to moles of given. Going from moles of given to moles of unknown requires us kind of to have a leap of faith going from an area where we know some information are given information to an area where we know nothing at all our unknown information because of this leap of faith. We call this the jump as we go from our given region to our unknown region.

Now remember, with stoichiometry, when we go and make this jump, we have to do a mole to mole comparison. So we use the coefficients in the balanced equation. From this point, if we know the moles of our unknown, we can easily transform it into ions, atoms, formula units, molecules, or even back into grams. But now for the unknown.

If you're not quite familiar with the stoichiometric chart, make sure you go back and take a look at my videos on stoichiometry. This is where we first laid down the groundwork for our stoichiometric chart, and this is just a slight modification to that previous one. Now that we've seen the stoichiometric chart, we'll put into action as we start doing questions dealing with gas stoichiometry.

Gas Stoichiometry Calculation Example

So here it says what mass of silver oxide is produced when 384 milliliters of oxygen gas at 736mm of mercury and 25�C is reacted with excess solid silver. All right, so step one is we need to map out the portion of the stoichiometric chart you will use. They're giving us within this question, the volume of the gas, the pressure of the gas and its temperature. So we know with that information we can find the moles of our gas. Because moles equals pressure times volume divided by RT.

First we need to convert our pressure into atmospheres. Remember one atmosphere is 760 tors or 760mm mercury. Millimeters of mercury cancel out and when we do that we get 0.9684 atmospheres. Then we are going to take the volume, which is 0.384 liters. You're going to divide that by the R constant and then remember you add 273.15 to get 298.15 Kelvin from the temperature. This information here, what is it doing? It's converting the given quantity that we have these amounts into our moles of given. So when I plug all this in, my moles of Given for oxygen gas comes out to be 0.015199 moles of oxygen gas.

Now that we have moles of given, we go to step three. We do a mole to mold comparison to convert moles of given into the moles of our unknown. So we're going to say here moles of oxygen go on the bottom are unknown as what we're being asked to find, which is our silver oxide. At this point we need to do a mole to mole comparison which says that for every one mole of oxygen gas we have two moles of silver oxide. So moles of oxygen gas cancel out and now we have moles of silver oxide.

Now Step 4, if necessary, convert the moles of unknown into the final desired units. All right, they're not asking us for moles of silver oxide. So I'm just continuing onward to where we get grams of silver oxide. So for everyone mole of silver oxide, the mass of two silvers and one oxygen has a combined mass of 231.7g of silver oxide. Moles of silver oxide cancel out. And now I'll have my final answer of silver oxide. So that comes out to be 7.0 grams of silver oxide here. Our answer has two significant figures because 25 has two significant figures.

Here we didn't have to do Step 5 because when step five it says recall. If you calculate more than one final amount, then you must compare them to determine the theoretical yield. Here we are only given amounts for oxygen gas South. Step 5 isn't necessary. Now if you don't remember the whole concept of theoretical yield, make sure you go back and take a look at my topic. Videos on theoretical yield. What does it mean and how does it relate to stoichiometry is explained in those series of videos. All right. So now that we've gotten our answer, this is the approach we need to take when it comes to gas stoichiometry.

The metabolic breakdown of glucose (C₆H₁₂O₆) (MW:180.156 g/mol) is given by the following equation:
C₆H₁₂O₆ (s) + 6 O₂ (g) → 6 CO₂ (g) + 6 H₂O (l)
Calculate the volume (in mL) of CO₂ produced at 34°C and 1728.9 torr when 231.88 g glucose is used up in the reaction.

• A. 8.6×10⁵ mL
• B. 6.8×10⁴ mL
• C. 8.6×10⁴ mL
• D. 4.3×10⁴ mL

Answer: (C)

The oxidation of phosphorus can be represented by the following equation:
P₄ (s) + 5 O₂ (g) → 2 P₂O₅ (g)
If 1.85 L of diphosphorus pentoxide form at a temperature of 50.0 ºC and 1.12 atm, what is the mass (in g) of phosphorus that reacted?

• A. 4.84 g
• B. 7.02 g
• C. 10.9 g
• D. 12.4 g
• E. 17.7 g

Answer: (A)

Determine the mass (in grams) of water formed when 15.3 L NH3 (at 298 K and 1.50 atm) is reacted with 21.7 L of O2 (at 323 K and 1.1 atm).
4 NH₃ (g) + 5 O₂ (g) → 4 NO (g) + 6 H₂O (g)

• A. 12 g
• B. 19 g
• C. 25 g
• D. 38 g
• E. 51 g

Answer: (B)

Here’s what students ask on this topic:

How to do gas stoichiometry?

Gas stoichiometry involves calculating the volumes, pressures, or temperatures of gases involved in a chemical reaction using the ideal gas law and the stoichiometric coefficients from the balanced chemical equation.

Here’s a step-by-step guide:

1. Write and Balance the Equation: Ensure the chemical equation for the reaction is balanced. The coefficients will give you the ratio of moles of reactants to products.
2. Convert to Moles: If you're given volumes at STP (Standard Temperature and Pressure, which is 0°C and 1 atm), you can use the fact that one mole of an ideal gas occupies 22.4 liters at STP to convert volumes to moles. If conditions are not at STP, use the Ideal Gas Law ($PV=nRT$) to find moles.
3. Stoichiometric Calculations: Use the stoichiometric coefficients from the balanced equation to calculate the moles of the desired substance based on the moles of the given substance.
4. Convert Back to Desired Units: If you need to find the volume, and you’re at STP, multiply the moles by 22.4 L/mol. If not at STP, rearrange the Ideal Gas Law to solve for the desired variable ($P$, $V$, or $T$).
5. Account for Non-Ideal Conditions: If

What is gas stoichiometry?

Gas stoichiometry is the quantitative relationship between reactants and products in chemical reactions involving gases. It is based on the principles of the ideal gas law, which relates the volume, temperature, pressure, and amount of a gas. In gas stoichiometry, you use the molar volume of a gas at standard temperature and pressure (STP) to predict how much gas is produced or consumed in a reaction.

For example, at STP (0°C and 1 atm), one mole of any ideal gas occupies 22.4 liters. If you're given a chemical equation and the amount of one reactant in moles, you can calculate the volume of a gaseous product by using the stoichiometric coefficients from the balanced equation. This allows you to predict the volume of gas produced from a given amount of reactant or vice versa. It's a crucial concept for understanding how chemical reactions proceed and for practical applications like synthesizing chemicals in the lab or industrial processes.