Expressing Concentration by Mass Percent

- Hey, it's Professor Dave. Let's talk about mass percent. 

- (SINGING) He knows a lot about the science stuff, Professor Dave explains. 

- In chemistry, we know that we can create a solution by placing some solute into some solvent. And we know that the solution will be of some concentration depending on how much solute there is relative to the amount of solvent. If there is very little solute we say the solution is dilute if there is a lot we say the solution is concentrated we've learned about how to use molarity to report concentrations, which is expressed in moles solute per liter of solution. But we will also sometimes want to describe concentrations in terms of mass, as for certain situations in the laboratory or for everyday substances, this will be an easier way to measure and report concentration. 

The mass percentage of a component of a solution is the ratio of the component's mass to the solution's total mass expressed as a percentage, which is shown by this equation. Mass will often just be referred to by listing a substance followed by a percent symbol. And the assumption is typically that it is mass that is being described. For example, we may describe the concentration of household cleaning products this way, since the majority of the people who use such products are not scientists and are unlikely to know about moles or molarity. So this is a more practical way for the public to conceptualize concentration, which is a concept that will be important to everybody. 

Take, for example, a typical bottle of bleach. The active ingredient in bleach is sodium hypochlorite. And we might commonly see the concentration on the bottle listed as being 7.4%. That means that this is an aqueous solution consisting of 7.4% sodium hypochlorite by mass. In other words, if there were a total of 100 grams of bleach, 7.4 of those grams would be sodium hypochlorite and the rest would be water. In order to make sure we can calculate mass percentage let's look at another household cleaning product. 

Let's say a bottle of bathroom tile cleaner contains 165 grams of hydrochloric acid and 790 grams of water. What would be the percent mass of hydrochloric acid in this cleaner? Using our equation, we can plug in the mass of the component, in this case HCl, and the mass of the solution, which would be the sum of these masses. Then we divide and multiply the result by 100. We should get 17.2% as our answer. This is the mass percent of hydrochloric acid in the product, which we can then compare to other products if necessary. 

As we said, this concept is also useful in the lab because we won't always want to have to do stoichiometric calculations. If we want to make an aqueous solution that is 10% sodium hydroxide by mass, we could measure out 10 grams of sodium hydroxide pellets on a balance, place that in a flask, and then simply add 90 milliliters of water, since water is precisely 1 gram per milliliter. 10 grams of sodium hydroxide plus 90 grams of water gives us a 100-gram solution that is 10% sodium hydroxide by mass, no calculations necessary. 

So we can see that this concept has a wide variety of applications both for the public and for industry let's check comprehension. 

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