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Molar Solutions with percent concentrations

  • #1
1. The molecular weight of NaCl is 58.54. A 1 M solution of NaCl has 58.54 g NaCl dissolved in distilled water to a total volume of 1000 mL or 1 L. a .1 M solution has .854 g/L. A .2 M solution has 11.71 g/L.

Qustion 1 - What is the molarity of a 5% w/v NaCl solution?

Question 2 - What is the percent concentration of a 2M NaCl solution?

Question 3 - The molecular weight of glucose is 180.16. Describe how you would make up 100 mL of a .5M solution.




2. The only equation I have come up with is Molarity*Volume = grams/molecular weight. But I have no clue if that is relevant or what to do with it.



3. All I have been able to do is verify what the information gives me such as:

1 x 1 = 58.54/58.54

.1 x 1 = 5.84/58.54

.2 x 1 = 11.71/58.54

But I don't know where percentage comes into this at all. I haven't been able to find any examples on the internet that show me how to even begin solving these questions.
 

Answers and Replies

  • #2
epenguin
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1. The molecular weight of NaCl is 58.54. A 1 M solution of NaCl has 58.54 g NaCl dissolved in distilled water to a total volume of 1000 mL or 1 L. a .1 M solution has .854 g/L. A .2 M solution has 11.71 g/L.


The 0.1 M solution must be 5.854 g/L not .854 g/L.

Do I need to explain that 5% w/v means 5 g/L?

You can do the first two questions without even knowing what M or molecular weight even mean, by simple proportions.

Though you cannot get anywhere in chemistry without knowing what they mean - you must surely find something in your textbook, course notes or internet.
 
  • #3
Sorry, yes it is 5.854 g/L.

For the first question would the answer be .08 M then? The second question confuses me though...I get 117.08 as the answer but how does that make sense as a percentage?

And for whatever reason I've got to do this worksheet for my biology class...I'm not sure why and the last time I studied chemistry was 4 years ago so I have very little idea about what all of this is and my teacher didn't give us any instruction other than to complete it.

Thanks for the response.
 
  • #4
epenguin
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Sorry, yes it is 5.854 g/L.

For the first question would the answer be .08 M then? The second question confuses me though...I get 117.08 as the answer but how does that make sense as a percentage?

And for whatever reason I've got to do this worksheet for my biology class...I'm not sure why and the last time I studied chemistry was 4 years ago so I have very little idea about what all of this is and my teacher didn't give us any instruction other than to complete it.

Thanks for the response.
:blushing:
I am sorry, of course 5% w/v means 5g/100 ml i.e. 50g/l
 
  • #5
I'm really confused.

For question one, does:

M = (50 g/l)/(58.54)

?
 
  • #6
epenguin
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Even correcting for that mistake your first answer to Q1 would not have been right.

For you second answer 117.08 is the right number, but pure numbers do not mean anything in this sort of question. It is 117.08 g/l. Not hard to work out how many g per 100 ml which is the w/v %. Again quote the units.

And since your information was given to 4 significant figures, the answers can be too.

Your teacher probably thinks the relevant concepts are so elementary that even someone who has forgotten most of his chemistry still knows them, so I recommend you revise moles, molarity and why they are used.
 
  • #7
epenguin
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I'm really confused.

For question one, does:

M = (50 g/l)/(58.54)

?
Sort of, the molarity is 50/58.54, the solution is (50/58.54) M.
 
  • #8
So for question one the answer is 50/58.54 M, which is .854 M.

For question two, I did (117.08 x 100)/1000 and got 11.7%.

For the third question I did:

(100/1000)*(.5) = x/180.16

and got 9 grams(?). So would that mean dissolving 9 grams of glucose in the 100 mL of solution? Or am I completely wrong?
 
  • #9
epenguin
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So for question one the answer is 50/58.54 M, which is .854 M.

For question two, I did (117.08 x 100)/1000 and got 11.7%.

For the third question I did:

(100/1000)*(.5) = x/180.16

and got 9 grams(?). So would that mean dissolving 9 grams of glucose in the 100 mL of solution? Or am I completely wrong?
Q1 is right.

also agree with Q2

Q3 9 g near enough. I get 9.01 g .

Not completely wrong - wording makes me not sure what you meant. You are making the solution by dissolving the glucose in water not in solution. What you should do to be accurate is add water to your glucose until the total volume is 100 ml. It says w/v - weight/volume. That does not make exactly 100 ml of water, though not far off. You can't practically calculate how much water, but you don't need to.
 
  • #10
Ok I think I understand it all now. Thank you for all of your help!
 

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