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### Course: AP®︎/College Chemistry>Unit 1

Lesson 4: Composition of mixtures

# Worked example: Calculating the mass of a substance in a mixture

Elemental analysis can be used to determine the amounts of substances in a mixture. For example, if elemental analysis tells us that a potassium supplement contains 22% K by mass, and we know that the K is present as KCl, we can calculate the grams of KCl in the supplement. Created by Sal Khan.

## Want to join the conversation?

• At , why does 1 mole of K equal 1 mole of KCl? Shouldn't 1 mole of K and 1 mole of Cl equal 2 moles of KCl?
• A mole tells you the amount of something, in units of Avogadro's number. It doesn't matter how big or small the molecule is, just how many of them there are. Here, it takes 1 mole of K and 1 mole of Cl to make 1 mole of KCl because KCl is a bigger molecule than K or Cl and one molecule needs exactly 1 K and 1 Cl atom. If you have 2 moles of KCl in a substance, you'd have 2 moles of K, not 1, because each KCl molecule contains 1 potassium atom.
• I'm so confused... why doesn't all the KCl equal 0.45g?
And what constitutes the other mass?
• Well KCl is basically just salt so ingesting all that salt at once isn't the most pleasant. So pill manufacturers put in other ingredients to make these sort of things more palatable. This includes things like sugars and food dyes often. If the supplement comes in some kind of starch casing that will also contribute to the mass.

Hope that helps.
• Is the reason the final answer isn’t equal to the total mass of the supplement because the supplement isn’t just KCl?
• Correct. A supplement pill's mass isn't entirely the desired nutrient's mass. It also includes the masses of the other ingredients such as the pill's casing, sugars, and dyes.

Hope that helps.
• I am thoroughly confused. I really don't get the stuff about moles or how to use them. Can anyone please clarify that for me?
• A mole of something is an Avogadro’s number of that thing, where Avogadro’s number is 6.022x10^(23). In the same way a dozen of something is 12 of that thing. So if I had a dozen atoms, I would have 12 atoms. If I had a mole of atoms, I would have 6.022x10^(23) atoms.

Molar mass of a chemical is the mass, in grams, one mole that that chemical has. And it usually is in units of grams per mole, or g/mol. So here the molar mass of potassium chloride is 74.55 g/mol, which means that one mole of potassium chloride, 6.022x10^(23) potassium chlorides, has a mass of 74.55 grams.

Hope that helps.
• why 1 mole KCl is equal to 1 mole K (since sal writes 1 mole KCl/1 mole K)?

am I right that this idea comes bcs KCl always contains atoms (no matter atoms K or Cl)?, so when I take 1 mole of KCl, it always counted as 6,22x10^23 atom (1 mole)?
• Let's remember that mole e quantity measurement not a mass measurement, it means that, if I have a compound made of potassium and chlorine and the mass of the 22% of K is present in the supplement as KCl i have the same mole of both, cause for each molecule of KCl e have one potassium, lets callback the fact that mole is a quantity measurement
• Multiplying the mass of potassium by the percent present give us the grams of what?
(1 vote)
• The 0.450 gram isn’t the mass of potassium, it’s the mass of the entire potassium supplement. There are other ingredients in the supplement other than potassium.

So, multiplying the mass of the supplement by the mass percent of potassium yields the mass of potassium in the supplement.

Hope that helps.
• I want to understand this clearly. At first, the question states that there are 0.450 grams of a "potassium supplement". Then they gave us the percentage of potassium in it and the molar mass of KCl. But the question they ask is that "how many grams of KCl are in the supplement?" So, starting with the first line, what exactly is the potassium supplement? I thought it was KCl.
• The potassium supplement is not entirely potassium chloride (KCl). As a potassium supplement, KCl would be the active ingredient since that contains the potassium. However, medication contains additional inactive ingredients which also contributes to the mass. These inactive ingredients can include sugar to make essentially a salt tablet more appetizing, the casing of the supplement to keep the pill intact, or a food dye.

If the total mass of the supplement is 0.450 g, and the mass of KCl is 0.189 g, then only 42 % of the supplement contains the active ingredient while the remaining 58 % are the inactive ingredients.

Hope that helps.
• Would 0.189g of K not be rounded to two sig figs (0.19g) because 0.099 is two sig figs, not three?
(1 vote)
• Sal forgot to include all the digits for the 22.0% calculation around . And of course 22.0% has three sig figs which should have resulted in a product with three sigs as well (since the 0.450 g had three sig figs too). And so the answer should have been 0.0990 g. Which means his final answer of 0.189 g is correct and has the correct number of sig figs even if he didn't show it correctly in his work.

Hope that helps.
• why are 3 sig figs used in the final answer and not 2? since we are multiplying 0.099, which only has 2 sig figs, do we not take that value into account for the final answer, since it was calculated from the given information?
(1 vote)
• I've explained this before on this video before, but Sal made a mistake at by not including an extra zero at the end of 0.099 g. If Sal begins with 0.450 g and he multiplies it by 22% (0.22), which is an exact number, then he should retain the same number of sig figs from the 0.450, which is three here. So technically Sal should be doing the last calculation using 0.0990 g.

Hope that helps.