Solutions: Mass Percent - Online Tutor, Practice Problems & Exam Prep

Mass percent is the ratio of the mass of a solute to the total mass of a solution, expressed as a percentage. For example, a 23% NaOH solution contains 23 grams of NaOH in 100 grams of solution, with 77 grams being the solvent. Understanding this concept is crucial for calculations involving solute and solvent components, as it lays the foundation for further studies in chemistry, including topics like molarity and solution concentration.

Solutions: Mass Percent Concept 1

Mass or weight percent is the percentage of a given element or compound within a solution. Now, we're going to say that mass percent can be seen as the mass component, which is usually the mass of our solute divided by the total mass of our solution times 100. For example, if we're given 23% NaOH, this means that we have 23%, which translates to 23 grams of NaOH over 100 grams of solution. Because we're dealing with the percentage, we always assume it's out of 100 grams.

We can further expand on this and say that we have 23 grams of NaOH still on top and remember, a solution is made up of solute plus solvent. So, that'd be 23 grams still of NaOH plus 100 minus 23, which gives us 77. 77 grams of our solvent. Within our calculations, it's important that you are able to see that mass percent can be broken down like this, and it can further be expanded where we look at both the solute and solvent components individually.

Keep this in mind as we approach more questions dealing with mass percent. Now, look at example 1. Take a look at it. Once you do, attempt it on your own. If you get stuck, move on to the next video and see how I approach example 1.

Solutions: Mass Percent Example 1

So here it states, calculate the amount of water in kilograms that must be added to 12 grams of urea in the preparation of an 18.3% by mass solution. Here the molar mass of urea is given as 60.055 grams per mole. Alright, so we need to determine what the kilograms of water are. And we're dealing here with mass percent. So, mass percent would equal the grams of our solute, so mass of or grams of solute divided by grams of solution times 100. Pulling in what we know. Well, we know that the percentage is 18.3%, the mass of our solute is this urea here and it's still down here as well. We don't know what the mass of our water is, so that's going to be our x, and this is getting multiplied by 100. Alright. So what we're going to do here is we're going to realize that we need to cross multiply, well, we need to multiply both sides here by 12.0 + x. So this cancels out with this. So now we have 18.3 times (12 + x) here in parenthesis. This 12 and this 100 are multiplying each other to give us 1200. Next, what we're gonna do is we can solve this in 2 ways. Either we can divide out the 18.3 right now or we can distribute it. It's up to you which method you want to use. So here when I distribute it, I get 219.6 + 18.3x equals 1200. Subtract out 219.6 here. And then I'm gonna get here, 18.3x equals 980.4. Divide both sides here by 18.3. X equals the mass of our water which comes out to 53.6 grams of H₂O. But we don't want the answer at the end of being grams, we want it to be in kilograms, so we do one more conversion. For every 1 kilogram, it's equal to 1,000 grams. So, that gives me 0.0536 kilograms of water. So, just remember in this question we're talking about mass percent so use that formula to isolate our variable, which in this case is grams of water. Once we do that, convert it to kilograms to get our final answer. Now that you've seen example 1, attempt to do the next example on your own, come back and see if your answer matches up with mine.

Solutions: Mass Percent Example 2

A solution was prepared by dissolving 51 grams of KBr, Potassium Bromide, in 310 ml of water. Calculate the mass percent of potassium bromide in the solution. Alright, so that's going to be mass percent equals the grams of our solute which is KBr divided by grams of solution. Here our solution is made up of our solute KBr and our solvent water. So, we need grams of KBr plus grams of water and this gets multiplied by 100. We already know the grams of KBr so we can immediately plug those in. So, that's 51 grams of KBr here on top and here on the bottom. Now, we need to determine the grams of water, but instead, we're given milliliters. Remember that water is roughly about 1 in terms of its density. Water is 1 gram per 1 milliliter. We use that information to help us get the grams of water. Now, this is a piece of information you just have to remember. The density of water is around roughly 1. So that's 310 grams that we have of water. So, plug that in and this gets multiplied by 100 and it gives us at the end 14.1%. So, this question wasn't that bad, you just had to remember the density of water is 1 and use that to find its grams. Once you find all the components, plug it into the formula to get our final answer.

Mass percent = 51 grams of KBr 51 + 310 grams of water × 100

Here’s what students ask on this topic:

What is mass percent in chemistry?

Mass percent, also known as weight percent, is a way of expressing the concentration of a component in a mixture or solution. It is calculated as the mass of the solute divided by the total mass of the solution, multiplied by 100. The formula is:

$\frac{\mathrm{mass} \mathrm{of} \mathrm{solute}}{\mathrm{total} \mathrm{mass} \mathrm{of} \mathrm{solution}} \times 100$

For example, a 23% NaOH solution means there are 23 grams of NaOH in 100 grams of the solution.

How do you calculate the mass percent of a solution?

To calculate the mass percent of a solution, follow these steps:

1. Determine the mass of the solute (the substance being dissolved).
2. Determine the total mass of the solution (solute + solvent).
3. Use the formula:

$\frac{\mathrm{mass} \mathrm{of} \mathrm{solute}}{\mathrm{total} \mathrm{mass} \mathrm{of} \mathrm{solution}} \times 100$

For example, if you have 23 grams of NaOH in 100 grams of solution, the mass percent is:

$\frac{23}{100} \times 100 = 23 \%$

What is the difference between mass percent and molarity?

Mass percent and molarity are both measures of concentration, but they are used differently:

• Mass Percent: This is the mass of the solute divided by the total mass of the solution, multiplied by 100. It is expressed as a percentage.
• Molarity: This is the number of moles of solute per liter of solution. It is expressed in moles per liter (M).

For example, a 23% NaOH solution means there are 23 grams of NaOH in 100 grams of solution. In contrast, a 1 M NaOH solution means there is 1 mole of NaOH in 1 liter of solution.

How do you convert mass percent to molarity?

To convert mass percent to molarity, follow these steps:

1. Determine the mass percent of the solute.
2. Calculate the mass of the solute in grams per 100 grams of solution.
3. Convert the mass of the solute to moles using its molar mass.
4. Determine the volume of the solution in liters (assuming the density if not given).
5. Use the formula:

$\frac{\mathrm{moles} \mathrm{of} \mathrm{solute}}{\mathrm{volume} \mathrm{of} \mathrm{solution} \mathrm{(L)}}$

For example, for a 23% NaOH solution, if the density is 1.2 g/mL, you can find the molarity by converting grams to moles and then dividing by the volume in liters.

Why is understanding mass percent important in chemistry?

Understanding mass percent is crucial in chemistry for several reasons:

• Concentration Calculations: It helps in determining the concentration of solutions, which is essential for reactions and experiments.
• Stoichiometry: It aids in stoichiometric calculations, allowing chemists to predict the amounts of reactants and products.
• Solution Preparation: It is used in preparing solutions with precise concentrations for various applications.
• Quality Control: It ensures the consistency and quality of chemical products in industries.

Overall, mass percent is a fundamental concept that underpins many aspects of chemical analysis and synthesis.