the pipette needs to be rinsed with the solution it is going to be filled with , the burette is rinsed with the solution and distilled water and the conical flask is rinsed with distilled water?
This is a very good rote memorisation of material, but it doesn't attack the heart of the question - which glassware should you rinse in which way to preserve the concentration or the number of moles? Firstly to do this, we need to answer what the role of each piece of glassware is:
a) The burette. It must deliver a compound of
exact concentration.
b) The pipette. It must deliver a compound of
exact concentration.
c) The conical flask. It must hold a compound of
exact quantity.
... but why? Well, let's go back to first principles. We want to know the concentration of some acid or base X. We're going to do that by reacting X with a base or acid, Y. That means we're going to have to use the equation:
X + Y ----> some salt + H
2O
Knowing that, this means that wherever the reaction takes place, it is VITAL that the moles in that place don't change. Because if they do, then we're going to have an issue. So, whatever we rinse the conical flask with, we have to make sure that what we've rinsed with DOES NOT change the amount of moles of the flask. This also means that whatever we deliver the acid or base with (the pipette or burette) must deliver the same amount of mole each time.
But the problem with the burette and pipette is we aren't delivering an amount of mole - we're delivering a VOLUME. Okay, so how do we compare the volume to mole? By the equation:
From this, you can see quite easily - if volume is going to stay the same EVERY TIME, then we also need the CONCENTRATION to stay the same EVERY TIME, or the mole will change. That means that whatever we rinse the burette and pipette with, they MUST make it so the concentration being delivered doesn't change, or else that will mean that the amount of mole being delivered will change.
Okay, so with this, it's important to know that WHATEVER you rinse the glassware with, then droplets of that is going to be leftover in the glassware. So, let's work with the following example: We are going to be titrating hydrochloric acid against 10 mL of 0.1M sodium hydroxide
The buretteSo first we need to identify what is going INTO the burette. In this case, the known volume is the sodium hydroxide, so that means the HCl will be going into the burette. Remember - the burette is always the one with the unknown volume. So, let's consider what happens if you rinse with each material:
- If you rinse with sodium hydroxide, then there will be droplets of base left in the burette. If you then add acid to the burette, the base droplets will change the concentration of the acid, so it is no longer exact. Remembering from above - if the concentration is different, but the volume stays the same, then the amount of mole delivered will be different - which is what we want to avoid.
- If you rinse with hydrochloric acid, then there will be droplets of acid left in the burette. If you then add acid to the burette, the droplets will combine - but the concentration won't change, because the concentration of the acid and the concentration of those droplets is the same. The concentration is still exact. Remembering from above - if the concentration is different, but the volume stays the same, then the amount of mole delivered will be different - which is what we want to avoid.
- If you rinse with DI water, then there will be droplets of water left in the burette. If you then acid to the burette, the droplets will dilute the concentration of hydrochloric acid, because the concentration of acid in the droplets is different to the concentration of acid in the solution. The concentration is no longer exact.
Let's go through this thought process for the other pieces of glassware:
The pipetteSo first we need to identify what is going INTO the pipette. In this case, the known volume is the sodium hydroxide, so that must be what the pipette delivers. Remember - the pipette is always the one with the known volume. So, let's consider what happens if you rinse with each material:
- If you rinse with sodium hydroxide, then there will be droplets of base left in the pipette. If you then add base to the burette, the base droplets will mix with the solution - but because the concentration of them both is the same, again, the concentration remains exact.
- If you rinse with hydrochloric acid, then there will be droplets of acid left in the pipette. If you then add base to the burette, the acid droplets will neutralise part of the base solution. The concentration will no longer be exact.
- If you rinse with DI water, then there will be droplets of water left in the pipette. If you then base to the burette, the droplets will dilute the concentration of sodium hydroxide, because the concentration of base in the droplets is different to the concentration of base in the solution. The concentration is no longer exact.
The conical flaskSo first we need to identify what is going INTO the conical. In this case, the known volume is the sodium hydroxide, so that means the sodium hydroxide will be going into the conical. Remember - the conical always initally holds what was in the pipette. So, let's consider what happens if you rinse with each material:
- If you rinse with sodium hydroxide, then there will be droplets of base left in the conical. If you then add base from the pipette to the conical, the base droplets will add extra moles to the conical. The quantity is no longer exact.
- If you rinse with hydrochloric acid, then there will be droplets of acid left in the conical. If you then add base from the pipette to the conical, the droplets will combine, and some of the moles of the acid will react with the base, reducing the quantity of base that the pipette delivered. The quantity is no longer exact.
- If you rinse with DI water, then there will be droplets of water left in the conical. If you then base from the pipette to the conical, then the water will dilute the amount of base of added - but the number of moles in the conical will be the same. The quantity is still exact.
Does knowing all of this change the answer you initially gave?