Homework Question G5

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Homework Question G5

Postby chrisavalos-2L » Thu Oct 04, 2018 6:55 pm

A student prepared a solution of sodium carbonate by adding 2.111g of the solid to a 250 mL volumetric flask 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 flask to obtain (a) 2.15 mmol Na+; (b) 4.98 mmol CO32-; (c)50.0 mg Na2CO3?

I was wondering what the first step would be to this equation, after I have calculated the molar mass of sodium carbonate? Would I divide it by the amount of liters of water added?

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Re: Homework Question G5

Postby Brandon_Tran_2E » Thu Oct 04, 2018 8:44 pm

Hey, this question is a bit tricky and I figured it out after Lavelle showed me through his office hours. First you would take your 2.111 grams of Na2CO3 and convert it to moles using the molar mass. Then take the moles and divide it by .250L to get the concentration of Na2CO3. The question is asking what volume is needed to obtain a) 2.15 mmol Na+ . So you take your .0215 moles of Na+ and divide it by the concentration of Na2CO3 to get liters in the end, but here's the tricky part: Since Na2CO3 has 2 Na+ ions, the concentration of Na+ must be double of the Na2CO3 concentration. The last step is taking .0215 moles of Na+ and dividing it by 2 x (concentration of Na2CO3 you calculated).

If you need the actual work of the problem just message again.

Brian Chhoy 4I
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Re: Homework Question G5

Postby Brian Chhoy 4I » Thu Oct 04, 2018 8:51 pm

The way I would approach this problem would to at first convert the mass of sodium carbonate to moles of the compound, and then divide this by .25 L to get the molarity of the solution (0.0796 M). Now for each question that they are asking for, start by converting the mmoles to moles by dividing by 1000 and then use the mole ratios to find the moles of sodium carbonate that are contained in the amount stated. For example, for (a) 2 moles of the sodium ion are found in 1 mole of sodium carbonate. In the end, divide by the molarity to cancel the unit of moles and you are left with how much volume of the solution you should use to get the amount specified. (a) V= 0.01349 L

Ashley Odibo Dis3E
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Re: Homework Question G5

Postby Ashley Odibo Dis3E » Fri Oct 05, 2018 12:12 am

Because the problem started out in ml do we have to convert our answers back to ml or would we be fine keeping our final answer in L?

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