Stoichiometry - Online Tutor, Practice Problems & Exam Prep

Stoichiometry is the study of the quantitative relationships in a balanced chemical equation, allowing us to predict the amount of product formed from given reactants or vice versa. To perform stoichiometric calculations, one must first ensure the chemical equation is balanced. The process involves converting the given quantity of a substance, which could be in grams, ions, atoms, formula units, or molecules, into moles (the 'Given Information'). Using the stoichiometric chart, we move from the given information to the 'Unknown Information' through a mole-to-mole comparison, guided by the coefficients in the balanced equation. This step is crucial and is often referred to as the "jump." Depending on the question, the calculation may stop at moles of the unknown substance or continue to convert back to grams, ions, atoms, formula units, or molecules of the unknown. This systematic approach is essential for accurately determining the mass or number of particles of a product or reactant in a chemical reaction.

Stoichiometry

When it comes to chemical equations, it becomes important to first balance your chemical equation because this will later on lead us to our understanding of stoichiometry. Now, stoichiometry deals with the numerical relationship between compounds. In a balanced chemical equation, it allows us to determine the amount of products from reactants and vice versa.

For example, here we have our chemical equation that's balanced. We have two hydrogen gases reacting with one oxygen gas to produce 2 water molecules as gas. They're giving us 12.3 grams of H₂, and they're asking us to determine the grams of H₂O gas produced. This is what stoichiometry is. They're giving us a balanced chemical equation. They're giving us information on one of the compounds within this balanced equation and asking us to find another compound within that same balanced equation.

Now that we know what stoichiometry is, how exactly do we do it? Well, if you click on the next video, we'll take a look at the procedures that you'll need to employ in order to do any type of stoichiometric calculation.

Stoichiometry

So as we said, we're given one 2.3 grams of H₂ and asked to find how many grams of H₂O would form. The way we're able to do this is through stoichiometry and to find how many grams we'd use the stoichiometric chart. Now the chart uses the given quantity of a compound to determine the unknown quantity of another compound. The way it works is we're starting here on the left side, and on the left side the information they give to us is called the given information.

Now this information that's given to us can be presented as grams, and if it's presented as grams, then we can just do a conversion to get to moles. So we can go from grams of given to moles of given. Now, besides giving it to us in grams, it could be given to us in ions, atoms, formal units or molecules of given. Again, we're heading towards moles of given. So our basic movement is going from the left side of the chart towards the right side of the chart to our unknown information.

Now that we have our moles of Given, we'd have to basically take a jump, a leap of faith from an area where we know information because it's given to us to an area where we know nothing at all. So it's unknown to us. And this jumper leap of faith that we're going to take, we call it the jump. To be able to do this jump, you have to do a mole to mole comparison and in a multiple comparison use the coefficients in the balanced equation. That's why it's incredibly important you first have a balanced equation before you even attempt to do stoichiometry.

Once we've gone from moles of given to moles of unknown and then we can either stop there if they want us to find the answer in moles, or continue onward to ions, atoms, formula units or molecules of unknown or to grams of unknown. In this case, from the example that we have up above of our equation, we would say that this is the grams given to us. So this would be our grams of given and we'd have to basically take the trip of grams of given to moles of given to moles of. Our unknown are unknown. Is what we're looking for, which would be water and then we'd have to stop here at grams of unknown.

That's the path we'd have to take. We utilize the stoichiometric chart to help us answer any type of stoichiometric question. Now that we've seen this chart, move on to our example question and let's put it to work, put it to practice in terms of this stoichiometric chart.

Stoichiometry

Here in this question, it asks how many grams of Nitrogen Monoxide are produced when 15 grams of Nitrogen react. Alright, here is our balanced equation. That's great. Now, step 1 is to map out the portion of the stoichiometric chart you will see or will use. They're giving us here grams of N₂. So this is our grams of given. Look on the stoichiometric chart. Focus on the grams of given. They're asking us to find grams of nitrogen monoxide. So we have to basically walk towards it. We are going to go from grams of given, to moles of given. Remember, it's going to require us to take a leap of faith, a jump, where we will go to moles of unknown. From there, we can go to grams of unknown. This is the portion of our stoichiometric chart that we're going to utilize to find our answer.

Step 2, we're going to convert the given quantity into moles of given. Before we do that, remember if a compound is set to be in excess, then we just ignore it. So here, we're going to have 15 grams of N₂. Let's convert that into moles. We want to find moles, so moles go on top. You want to get rid of grams of N₂, so it goes on the bottom. Two Nitrogens together. Nitrogen weighs 14.01 grams according to the periodic table. Multiplying it by 2 gives me 28.02 grams, which is what we're going to place here.

Step 3. Do a mole to mole comparison to convert moles of given into moles of unknown. We want to get rid of moles of O₂ (circles) on the bottom. Our unknown is what we need to find, which is our nitrogen monoxide, so moles of NO on top. Remember, we're going to do a multiple comparison so we have to look at the coefficients of the balanced equation, which says that for every 1 mole of N₂, there are 2 moles of NO. We're now done with moles of given. We are now at moles of unknown.

If necessary, convert the moles of unknown into the final desired units. In the question, we need to find the grams of NO, not the moles of NO. One more step. We want to get rid of moles of NO, so put it down here: 1 mole of NO. Now, realize that when we do the mole-to-mole comparison, this is the only place where we use the coefficients from the balanced equation. After this, I go back to just being 1 mole. So, 1 mole of NO which is 1 Nitrogen, 1 Oxygen. When you add those together, it gives me 30.01 grams. Here, moles of NO cancel out, and I have my final answer which is grams of NO. In the question, 15.0 has 3 significant figures, so our answer should have 3 significant figures. So, this would be our final answer: 32.1 grams of NO.

The combination reaction for the formation of hydrazine is shown below:

4 NH₃ (g) + N₂ (g) → 3 N₂H₄ (l)

Determine the number of moles of hydrazine formed when 25.7 g ammonia reacts with excess nitrogen gas.

• A. 1.13 mol
• B. 0.567 mol
• C. 0.881 mol
• D. 2.81 mol

The reaction between solid phosphorus and fluorine is given below:

P₄ (s) + 12 F₂ (g) → 4 PF₆ (aq)

If 6.92 x 10^-5 molecules of solid phosphorus react to completion, how many grams of phosphorus hexafluoride would be produced?

• A. 5.03 x 10^-26 g
• B. 7.14 x 10^-26 g
• C. 8.19 x 10^-26 g
• D. 6.65 x 10^-26 g

Determine the density of 50.0 mL iron (II) bromide when the following chemical reaction produced 112.7 g iron (II) phosphate.

2 K₃PO₄ (aq) + 3 FeBr₂ (aq) → Fe₃(PO₄)₂ (s) + 6 KBr (aq)

• A. 2.31 g/mL
• B. 6.59 g/mL
• C. 4.08 g/mL
• D. 3.95 g/mL

Here’s what students ask on this topic:

What is stoichiometry?

Stoichiometry is a section of chemistry that involves the quantitative relationships between the reactants and products in a chemical reaction. It is based on the conservation of mass and the concept that elements are neither created nor destroyed in chemical reactions. Stoichiometry allows chemists to predict the amounts of substances consumed and produced in a given reaction.

To perform stoichiometric calculations, you'll need to use the balanced chemical equation for the reaction. A balanced equation shows the correct proportions of reactants and products, adhering to the law of conservation of mass. From this, you can determine the mole ratio between the substances, which then enables you to calculate how much of one substance is needed to react with a given amount of another, or how much product will be formed from given reactants. This is essential for tasks such as scaling up reactions for industrial purposes, determining yields in laboratory experiments, and analyzing the composition of samples in analytical chemistry.

How to do stoichiometry?

Stoichiometry is a method in chemistry that relates the quantities of substances in a chemical reaction. To perform stoichiometry, follow these steps:

1. Write a Balanced Chemical Equation: Ensure the number of atoms for each element is the same on both sides of the equation.
2. Convert Units to Moles: Use the molar mass of the substances to convert grams to moles if necessary.
3. Use Mole Ratios: From the balanced equation, determine the mole ratio between the reactants and products.
4. Calculate Moles of Unknown: Using the mole ratio and the moles of the known substance, calculate the moles of the unknown substance.
5. Convert Moles to Desired Units: If the problem asks for grams or liters (for gases at STP), convert the moles of the unknown substance to these units using molar mass or molar volume (22.4 L/mol for gases at STP).

Remember, the key to stoichiometry is the balanced chemical equation and the mole ratio it provides, which serves as the conversion factor between reactants and products. Practice with various problems to become proficient in this method.

How is stoichiometry used to calculate the amount of product produced?

Stoichiometry is a branch of chemistry that deals with the quantitative relationships between the reactants and products in a chemical reaction. To calculate the amount of product produced using stoichiometry, you follow these steps:

1. Write down the balanced chemical equation for the reaction. This shows the ratio in which reactants combine and the products formed.
2. Determine the molar mass of the reactants and products using the periodic table.
3. Convert the mass of the reactants used to moles, using their molar mass.
4. Use the mole ratio from the balanced equation to calculate the number of moles of the product that should form.
5. Convert the moles of the product back to mass, if required, using the molar mass of the product.

This process allows you to predict the amount of product that will be produced from a given amount of reactants, assuming the reaction goes to completion and all conditions are ideal. It's a fundamental concept for understanding chemical reactions and is widely used in chemistry to ensure reactions are carried out with the correct proportions of reactants.

How to do mass-to-mass stoichiometry?

Mass-to-mass stoichiometry involves converting the mass of one substance in a chemical reaction to the mass of another substance using the mole ratio from the balanced chemical equation. Here's how to do it:

1. Write the balanced chemical equation for the reaction you are studying. This will provide you with the mole ratio of the reactants and products.
2. Convert the mass of the given substance to moles using its molar mass. The molar mass is the mass of one mole of a substance and can be found on the periodic table (in grams per mole).
3. Use the mole ratio to convert moles of the given substance to moles of the desired substance. The mole ratio comes from the coefficients of the substances in the balanced equation.
4. Convert the moles of the desired substance to mass using its molar mass. Multiply the number of moles by the molar mass of the substance to find the mass.

In summary, the process is: mass of substance A → moles of substance A → moles of substance B → mass of substance B. Always ensure that your units cancel out appropriately during each step, and check your work with dimensional analysis.