Fundamentals G5

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Chin_Alyssa_3I
Posts: 34
Joined: Fri Jul 22, 2016 3:00 am

Fundamentals G5

G.5 A student prepared a solution of sodium carbonate by adding 2.111 g of the solid to a 250.0-mL volumetric fl ask and adding water to the mark. Some of this solution was transferred to a buret. What volume of solution should the student transfer into a fl ask to obtain (a) 2.15 mmol Na+; (b) 4.98 mmol CO32-; (c) 50.0 mg Na2CO3?

So I started out be calculating the moles of Na2CO3 by dividing 2.111 g Na2CO3 with its molar mass (105.991g/mol) and got .01992 mol.

I then found the Molarity by dividing ,01992 mol with the volume (.250 L) and got .0796 mol/L ( or .0800 mol/L considering sig figs)... What do I do next?

mikezargari
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Re: Fundamentals G5

So the molarity of Na2CO3 is 0.08 M according to your calculations. Because for every one mole of Na2CO3 you have 2 moles of Na+1 multiply 0.08 M by 2 and that is the molarity of Na+1. Take 0.00215 mol/ 0.08 M times 2 and that should give you your answer. Makes sense too because units cancel out to give you liters.

Chin_Alyssa_3I
Posts: 34
Joined: Fri Jul 22, 2016 3:00 am

Re: Fundamentals G5

Okay make sense. So with that in mind then for part b, since for 1 mol of Na2CO3 there is only 1 mol CO32-, you would just divide the mol with .08 M, correct?

Shushanna S 3F
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Joined: Sat Jul 09, 2016 3:00 am

Re: Fundamentals G5

Yeah that's correct.

Yuchien Ma 2L
Posts: 46
Joined: Wed Sep 21, 2016 2:58 pm

Re: Fundamentals G5

Using the formula M1V1=M2V2 I set up the following:

(0.159338 mols Na+/L) (V1) = (2.15*10^-3 mols Na+/L) (0.250 L)

and solving for V1 I got:
V1= 3.373 * 10^-3 liters, 3.373 mL, which is different from the solutions manual (which is 13.5 mL).

Please help!

Chem_Mod
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Re: Fundamentals G5

For this particular problem, you do not use the equation M$M_{1}V_{1} = M_{2}V_{2}$ , because it does not involve a dilution. You are simply transferring a specific amount of the original solution to a new flask, but you do not add water to dilute it. Instead, the question is asking how much of the solution must you transfer to get the required number of moles. As mentioned in previous comments, you can get the answer by dividing the requested number of moles by molarity to get the volume.

Yuchien Ma 2L
Posts: 46
Joined: Wed Sep 21, 2016 2:58 pm

Re: Fundamentals G5

I finally got it!
Thanks for your help

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