Cheers. I am doing a hands on lab for intro to Chemistry. I basically have to do something seemingly simple. I pour 8.0 grams sugar, weighed, into a volumetric flask that is volume of 25mL. I add 15mL of water in then shake it up. I then pour 10 more mL. They want me to weigh the volumetric flask empty without the stopper. I then add the 8.0 g sugar and then the distilled water. After weighing the full flask, I have to determine the initial and final concentration. I have been looking up this formula but I see three different units are being used. This is not in molars. I assume I did the total volume right, which was 25 mL, I assume I did the concentration right, as I basically divided 8.0g by 25mL, then multiplied by 100%, which gave me 32. I am flabbergasted ont his initial and final concentration though. Any feedback on this? I'd appreciate it.

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    $\begingroup$ Your procedure as you state it here is different from what you describe in our comments to the answer by LigninPauling, where you say after mixing sugar with 15 mL of water, you filled up the solution to 25 mL with a transfer pipette. So in the end, you have 8.0 g sugar in 25 mL of solution (not 25 mL of water). $\endgroup$ – Karsten Theis Feb 15 '19 at 14:42
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    $\begingroup$ You said you measured the mass of the empty flask and the full flask. What were the values? $\endgroup$ – Karsten Theis Feb 15 '19 at 14:44
  • $\begingroup$ empty it weighed 22.5 g. sugar was 8.9 g. full with the sugar and remaining water up to the 25mL, the flask weighed 50.4g. $\endgroup$ – Stu Pedaso Feb 15 '19 at 15:38
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    $\begingroup$ Water is about 1.0 g/ml. 50.4 - 22.5 - 8.9 = 19.0 Thus you could only added about 19.0 ml of water into the 25 ml volumetric flask, not 25.0 ml. $\endgroup$ – MaxW Feb 15 '19 at 16:43
  • $\begingroup$ @Karsten Theis: According to OP's answer to your comments, OP has 8.9 g sugar in 25 mL of solution or 35.6% ($w/v$) concentration (not 32% as OP calculated). $\endgroup$ – Mathew Mahindaratne Feb 16 '19 at 1:33

Let's try and break this down.

We will assume that the mass of sugar was exactly 8.0 grams and that the volume of water was exactly 25.0 mL. Weighing the flask before and after the addition of the water would infact give you the volume of water added. 1 mL of water has a mass of 1 g and so the mass of the flask should have increased by 25 g.

For the concentration, it is unclear from your question what units were requested. If it is percent by mass then 32% sugar looks reasonable.

If it is requested in mass units per volume then perhaps g/L will suffice. You can calculate this for yourself quite easily.

Lastly, if molarity is required for your answer, this is slightly more involved. 'Common table sugar' (presumably what was used in this practical) is sucrose and a quick search will reveal its molar mass to be 342.3 g/mol. Therefore 25 g of sucrose equates to 0.0730 mol. Using this, the molar concentration may be worked out, taking care to note the units of molarity.

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  • $\begingroup$ Yes my bad. Basically, it was just a 25mL volumetric flask, I shook the distilled water and sugar til it fully mixed and then after entering the volume before and after adding the solution mix(8.0 g sugar and 25mL water) the final question is to give the initial concentration and the final concentration. The initial has a % sign next to it g/mL. The final has a percent sign next to it as well. I'm assuming later I will learn why they ask for an initial and final concentration but for now I guess idk why they would want the same thing written twice. $\endgroup$ – Stu Pedaso Feb 15 '19 at 12:22
  • $\begingroup$ In your question you mentioned adding 15 mL first and then afterwards adding another 10 mL. Was this included in the lab instructions? If so then perhaps the first could be the initial concentration they are asking for? $\endgroup$ – LigninPauling Feb 15 '19 at 12:38
  • $\begingroup$ They said 15mL because it would be easier to stir the solution before adding the other 10mL. I also thought that same thing but, the result was filling the 25mL flask with the 8.0g of sugar and then using the pipet to fill up to the 25mL mark after mixing the solution better. $\endgroup$ – Stu Pedaso Feb 15 '19 at 12:41

Since it is a lab for intro to Chemistry, this may directing to find basic knowledge of density and/or concentration, % concentration, etc. However, what puzzled me was the asking of intial concentration and final concentration. After reading all of comments and OP's answers, I thought this (see below) may be the question:

Weigh empty 25-mL volumetric flask first without the stopper. Then weigh $\pu{8.0 g}$ sugar and pour into the volumetric flask. Then, add $\pu{15 mL}$ of water to it and shake it up. Then, pour more water to the $\pu{25.0 mL}$ mark. Weigh the full flask again without the stopper. Determine the initial (after addition of $\pu{15 mL}$ of water) and final concentration (after addition of water to the $\pu{15 mL}$ mark).

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  • $\begingroup$ that is exactly right. Also, after I weigh everything, I pour the solution 0 into a 100mL empty beaker, then take my pippete serological and I take 2.5mL and put it into the emptied and cleaned volumetric flask, and then add 15 more mL of distilled water and re-weigh everything and then repeat the steps again with different denominations of distilled water. Then after, I make a graph and show the x and y axis for the mass and volume. $\endgroup$ – Stu Pedaso Feb 16 '19 at 5:45

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