If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

## AP®︎/College Chemistry

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

Lesson 4: Composition of mixtures

# Worked example: Analyzing the purity of a mixture

Elemental analysis can be used to analyze the purity of a sample. For example, a pure sample of NaCl should contain 61% chlorine by mass. If elemental analysis tells us that the sample actually contains 73% chlorine by mass, this suggests that our sample has been contaminated by a compound containing a higher mass percent of chlorine. Created by Sal Khan.

## Want to join the conversation?

• At , Sal says that you can figure out how much % of the sample is NaCl and LiCl based on the percentages of chlorine by mass(73%, 61%, and 84%) . How would you find how much % of the sample is NaCl and LiCl? • If the sample was pure NaCl, the % of chlorine by mass would be 61%. If it were pure LiCl, it would be 84%. We can use these two points to draw a line:
percentage chlorine by mass = 61% + 23% * percentage LiCl by mass
This becomes 73% = 61% + 23% * x
Solving for x gives x = 52%. So 52% of the sample by mass is LiCl and 48% is NaCl
• At , Sal says that you can figure out how much % of the sample is NaCl and LiCl based on the percentages of chlorine by mass(73%, 61%, and 84%) . How would you find how much % of the sample is NaCl and LiCl? • so it contains 73% chlorine by mass, i know we used the concept of averages to get the idea about which one was increasing the percent mass of Cl but like how can we be sure it is only LiCl, there could be some KCl in there too and since the mass ratio is almost 1:1 for KCl, it wouldnt drag the Cl ratio down too heavily anyway, and if we add enough LiCl eventually the ratio will just jump back up for Cl, am i right? if so then this is such a frustrating question as it is not being specific in details and expecting us to be sure about our answer, i really cant get how can one even know where to start in questions like this, so thats just adding to my irritation, can someone please help? • Good point. Sal assumes that the only two options are a pure sample of sodium chloride (only sodium chloride) or a mixture (two or more compounds) of sodium chloride and only one other compound. However it possible that it could be a mixture of three or four compounds.

If it contained NaCl, KCl, and LiCl, they would all effect the percentage of chloride in the sample.

Additionally, sodium iodide could be in there too. We're checking for chloride, and just because sodium iodide doesn't have any chloride, that wouldn't rule it out as being part of the mixture. It just wouldn't be detected if we checked only chloride content.

I guess we assume it could potentially only be a mixture of two compounds because of the wording of the question. "You suspect that it may have some NaI, KCl, or, LiCl as well.", with emphasis on the "or" part. It's saying that if indeed it is a mixture, it would only contain one of those three contaminants.

Hope that helps.
• So, just adding those percentages together for NaCl and LiCl 61+84 then we divide it by 2 which would give us the percentage in the question 72.5 which is almost 73%
Is that how to do it to make sure it’s correct?
(1 vote) • Pretty much, yes. If we have a mixture of sodium chloride and lithium chloride with % chlorines of 61% and 84%, then their mixture’s % chlorine will be somewhere between those two numbers. Using an arithmetic average though assumes that the two salts are mixed in equal amounts which isn’t always true. So it’s not that their average, 72.5%, is close to the samples chlorine %, 73%, it’s that the sample’s chlorine % is in between the pure salt’s chlorine %s.

Hope that helps.
• I'm confused about whether or not there is only LiCl and NaCl. This is because LiCl is more than 50% of the mixture, but the question says that the substance is mostly NaCl. Does this mean that there are more elements present? • But how can we be sure that both NaCl and LiCl in the sample are exactly 1 mole? If we assume there is just more NaCl in the sample (for instance 2 moles), but the amount of LiCl is 1 mole, while the percentage of Cl in NaCl stays the same ~61%, in total we would get something like this:

- Amount of Cl in NaCl = 71 g
- Amount of not Cl in NaCl = 46 g
- Amount of Cl in LiCl = 35.5 g
- Amount of not Cl in LiCl = 7 g

In total we would get this percentage of Cl in the sample:
- (71 + 7)/(71+7+35.5+46) = ~49%
Is there something wrong in this assumption?
(1 vote) • Since we’re using mass percent the amount of moles is irrelevant. If we assume we had pure sodium chloride, we would have the same 61% mass percent for the chlorine in the sample if we had 1 mole or 100 moles. Likewise, if the sample is contaminated with lithium chloride, be it 1 mole or 100 moles, it would increase the sample’s chlorine percent mass above 61%.

Hope that helps.
• i tried calculating the cl percentage by mass for the whole sample in the same way we did for each compound.
(35.45+35.45)/(22.99+35.45)+(6.94+35.45) = 0.7031 = 70% cl by mass in NaCl and LiCl. shouldnt it be 73%? why did i get 70%?
(1 vote) • if the percentage of nacl was 72% would that have made it a pure sample?  • Shouldn't the `Na%` also be counted when analyzing `NaI`?
If it's pure `NaCl`, then the mass percentage is equal to the relative atomic mass ratio, but when `NaI` is present, for the same amount of matter it's like replacing some `Cl` with `I`, so `Cl%` should decrease rather than stays the same. 