[roulette234]

Hi,

For an EDTA titration, we use 1.00 mL of seawater and 50.0 mL of deionized water, as well as buffer and Eriochrome black T indicator. To calculate the molarity, would we use 50 mL or 51 mL for the volume of the sample?

Also, another similar question: i.e. If we have a sample (solute) dissolved in 50 mL of 95% ethanol and 10 mL of deionized water. What is the volume of the sample? Would it be 60 mL?

[enahs]

Yes, 51ml and 60ml.

Molarity is by definition moles of solute per liter of solution. If you add more solution you change the molarity. Just remember to convert the ml to liters or you are getting millimolarity.

[roulette234]

so, molarity = moles of solute per liter of solution. But doesn't that mean we have to take into account how much buffer and how much indicator we added to the solution?

[enahs]

Your indicator will be two to three drops. If your equipment is accurate enough to measure ~1/4 of a milliliter then yes you should account for it, but since odds are it is not you can ignore it. And yes you should count the volume of buffer if you add some.

[Borek]

To calculate the molarity, would we use 50 mL or 51 mL for the volume of the sample?

Question is: calculate molarity of what? Original 1mL sample? Or titrated solution? In general, you are not interested in the titrated solution, as amounts of buffer/water/indicator added are approximate, so you don't know volume of your solution precise enough. However, regardless of the volume of titrated solution you can calculate what was exact amount of the substance (in grams or moles) in the original sample - and as you know original sample volume (1mL in your case) you can calculate exact concentration (in the seawater in this case).

Also, another similar question: i.e. If we have a sample (solute) dissolved in 50 mL of 95% ethanol and 10 mL of deionized water. What is the volume of the sample? Would it be 60 mL?

In general - no. Volumes are not additive.

50mL of 95% (w/w) ethanol + 10 mL of deionized water gives 58.8mL of 76% (w/w) solution.

[enahs]

Yeah Borek is right. I forgot to point out that odds are you want the molarity of the original solution, not the molarity of the final solutions (despite the fact that that is what you asked).