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We were told to rinse the buret with the base so that the concentration won't be messed up, but when it comes to the flask that is going to hold the acid, there is no need to rinse, because it would not affect the result if it has water in it.

Why is that? Wouldn't the acid be diluted a little bit and that throws off the accuracy?

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The volume and concentration of base equals the number of moles of base. This is use to determine the number of moles of acid in the flask. Since you are interested in the number of moles of acid in the flask, why would adding water (which doesn't have any acid) change the number of moles of acid?

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    $\begingroup$ Please note that the proper term for "number of moles" is amount of substance. The former would be the same as referring to the mass as "number of kilograms". $\endgroup$ – Martin - マーチン May 21 '19 at 21:33
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    $\begingroup$ For a first year student, referring specifically to number of moles probably carries more pedagogical value. "Amount of substance" may not expediently lead them to infer moles are the precise variable in question. Moles are not as much a practiced intuition as the more tangible kilograms that we often use growing up. The consistent English is lost on new students (even more experienced chemists like LDC3 don't consider it), and the explanation will be slightly more opaque for it. $\endgroup$ – Blaise May 21 '19 at 23:32
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Considering a direct titration:

Titration Set up

You will probably use a volumetric pipette to pass the acid solution to the erlenmeyer used on the titration, so the volume of acid is known (the exactly volume of the pipette).

Volumetric Pipette

Any distilled water you add to this erlenmeyer will change its volume, but won't change the amount of substance of acid inside it, neither the initial volume you added of the acid solution. Sometimes it's even recommended that you add some water to make the color change more visible.

The objective of the titration is to find the volume of the base (of known concentration) necessary to neutralize the acid, and therefore, since you know the volume of acid added in the erlenmeyer (the pipette volume), calculate the concentration of the acid solution.

Water in the glass of the buret can cause variations in the concentration of the base being used, reason why we rinse it with the base, so we have a good precision titration. The erlenmeyer can be rinsed only with distilled water, since the volume of acid solution used for the calculation is constant.

You will have a equation that looks like this:

$$V(\text{base}) \cdot c(\text{base}) = V(\text{acid}) \cdot c(\text{acid}) \cdot f$$

Where $V(\text{base})$ is the volume of base consumed in the titration, $c(\text{base})$ is the concentration of the base (which is known), $V(\text{acid})$ is the volume of the solution of acid added to the erlenmeyer (the volume of the pipette), $c(\text{acid})$ is the estimated concentration of the acid and $f$ is the correction coefficient (so the real acid concentration is $c(\text{acid}) \cdot f$).

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  • $\begingroup$ Please note that the proper term for "number of moles" is amount of substance. The former would be the same as referring to the mass as "number of kilograms". $\endgroup$ – Martin - マーチン May 21 '19 at 21:42
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Where concentration matters ( burette ) there is rinsing to achieve stable and constant concentration.

Where amounts matters ( flask ) there is no rinsing to achieve stable and constant amount.

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If you were to calculate the concentration of acid from the titration, and you add distilled water to the erlenmeyer flask, then yes, you would affect the [H3O+(aq)] since increasing volume with same number of moles decreases the concentration of the solute (or analyte) and thus increases the pH of the solution (because pH is -log(hydronium conc.)), ultimately affecting the color of the indicator since the indicator is affected by the pH of the solution. Since you're using the indicator to determine of the titration has reached its endpoint and therefore the predicted equivalence point, if the indicator is affected by the dilution, the entire result of the titration will be afflicted with error if you were to add distilled water into the erlenmeyer.

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  • $\begingroup$ Welcome to Chemistry.SE! Take the tour to get familiar with this site. Mathematical expressions and equations can be formatted using $\LaTeX$ syntax.Please note that the proper term for "number of moles" is amount of substance. The former would be the same as referring to the mass as "number of kilograms". I believe the error you make when taking a reading is worse than a little bit of dilution. $\endgroup$ – Martin - マーチン May 21 '19 at 21:35

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