[Luisa2901]

You received a water sample to determine its "hardness", expressed as the total amount of carbonates (such as CaCO₃). The sample solution is approximately 5 times more diluted than the EDTA solution.
a) Based on this information, and knowing that you have at your disposal volumetric pipettes of 10, 25, and 100 mL and graduated pipettes of 5, 20, and 50 mL, state the volume of the sample (Vs) that you would use for the determination (the burette used for the titration is 50 mL). Justify your answer.

I know that M_EDTAXV_EDTA=M_sxV_s

Let y be the molarity of the water sample:
5yx50=yxV_s

So it gives me that V_s= 250 ml

What am I doing wrong?

[Hunter2]

Easy  thinking. Your sample is 5 times diluted as your EDTA solution.
For 1 ml of water you need  how much of EDTA.
Your calculation is correct if you choose 50 ml EDTA, but to expensive and you would use the full volume of the burette.

Start with volume of water what you can easy pipette with the available Equipment.

[Luisa2901]

I would use a volumetric pipette, as they accurately transfer a specific volume.

Based on that, I calculate the volume of EDTA required in the titration.

For 10ml of water:
5yxV_EDTA=yx10  V_EDTA=2ml

For 25ml of water:
5yxV_EDTA=yx25  V_EDTA=5ml

For 100ml of water:
5yxV_EDTA=yx100  V_EDTA=20ml

So, I believe the 100ml volumetric pipette is better, as it gives us a larger margin for titration. Does it make sense?

[Hunter2]

Yes correct 100 ml is a good volume.

The rule is normal, the consumption should be about 1/3  of the volume of the burette . 50 ml  means about 16 ml.
For 100 ml  sample you would need approx  20 ml EDTA.
50 ml would be also okay. Means 10 ml EDTA.

[Borek]

The rule is normal, the consumption should be about 1/3  of the volume of the burette

Sounds off to me, the higher the volume the higher the relative accuracy.

I was taught to aim at around 80% of the burette - gives much lower error, and a reasonable reserve in case sample is larger than expected.

[Hunter2]

Mathematically correct.
But 80% is in my opinion almost the upper value. In this case consumption of approx.  40 ml. Means more expensive of consumed titrant and the sample volume has to be 200 ml using a 300 ml Erlenmeyer. This is already difficult to sway, if no magnetic stirrer is available.