AAMC 4 question 30

[oms47]

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Question:
How many grams KNO3 are in 100 mL of the student-prepared solution of KNO3(aq)?

Fairly straight forward question.

Looking at the passage, we are given a table of information. The one I picked was the molarity of the solution which was 2.241mol/L. Once you convert this to grams in 100 ml you get 22.41grams (answer choice B...which I picked).

The correct answer was 22.65 which comes from taking the solute concentration given in the table (226.5g/L).

Why is 22.41 grams wrong. I can see why 22.65 grams is right but can't see why 22.41 grams is wrong...

Thanks
~O

 

[pfaction]

If I remember right, I just did the density. You looked at molarity. Molarity was .2241 mol/0.1L!

.224(101) is 22.6 🙂

 

[MedPR]

Question:
How many grams KNO3 are in 100 mL of the student-prepared solution of KNO3(aq)?

Fairly straight forward question.

Looking at the passage, we are given a table of information. The one I picked was the molarity of the solution which was 2.241mol/L. Once you convert this to grams in 100 ml you get 22.41grams (answer choice B...which I picked).

The correct answer was 22.65 which comes from taking the solute concentration given in the table (226.5g/L).

Why is 22.41 grams wrong. I can see why 22.65 grams is right but can't see why 22.41 grams is wrong...

Thanks
~O

How did you get 22.41 grams? 2.241mol/L at 101g/mol. (2.241mol/L)(1L/10^3mL)(101g/mol)(100mL) = 22.63g.

 

[MedPR]

If I remember right, I just did the density. You looked at molarity. Molarity was .2241 mol/L!

.224(101) is 22.6 🙂

If the molarity was 0.2242mol/L you wouldn't have 22.6 grams. You would have 2.26 grams.

 

[pfaction]

Whoops, I meant, 2.241 mol/L -> .2241/100mL.

 

[folktale]

If I remember right, I just did the density. You looked at molarity. Molarity was .2241 mol/0.1L!

.224(101) is 22.6 🙂

I did the density for this question as well. Turned out using the molarity was a faster route, but it gave the same answer so 😀

 

[pfaction]

How so? Take the density and move it over a decimal point; no need to divide and then multiply or anything!

 

[folktale]

How so? Take the density and move it over a decimal point; no need to divide and then multiply or anything!

I might be mixing this question up with another one on the same passage. Not sure, but that was the case when I was trying to calculate.

 

[aspiringdoc09]

I missed this question too. At first I tried using the value the OP used but when I found it wasn't an answer choice I used the density. I ended up calculating incorrectly, so I just guess an answer due to time. I couldn't come back because I ran out of time at the end.

 

[MedPR]

How so? Take the density and move it over a decimal point; no need to divide and then multiply or anything!

It's the same calculation as using the molarity. Either way you do it, you have to figure out the molar mass of KNO3. When you see that it's a little greater than 100, you know that it will be a little more than the molarity with the decimal place moved.

 

[pfaction]

Fun fact; on AAMC5 (whited out so those who haven't taken it won't get spoiled)
There was a momentum question on which I rounded 110 to 100, 500/100 = 5. The answer was 4.6. I kicked myself. Rounding is my foe.