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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.

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• 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
• 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?
(1 vote) • 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.
• 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?
(1 vote) • 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? • 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.
(1 vote) • 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?
(1 vote) • 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?
(1 vote) • For the question: 1.02g of Mg supplement contains 25% Mg by mass also Mg is present in the supplement as MgO with a molar mass of 40.30g/mol. how many grams of MgO is in the magnesium supplement?

my approach to this question was somewhat intuitive and I was wondering what was off with my method since the question kept grading me wrong

25% of the sample is Mg which means that from 1.02g equivalent to 100%, Mg in grams in the sample is 0.255g

the relationship between Mg and MgO is 1 mol to 1 mol

- Mg which has an atomic mass of 24.31g/mol; meaning that 0.255g of Mg represents 0.01 mol of Mg and since the relationship with MgO is 1 to 1 then, Oxygen with an atomic mass of 16g/mol 0.01 mol will give us a corresponding weight in grams of 0.16g which in addition to the 0.255g of the Mg the total weight in grams of MgO in the supplement with a concentration of Mg 25% would be 0.415g

what is wrong with my approach that keeps grading me wrong? 