I'm sure that most people get it, but I'll try explain the way our teacher taught us to think about these.
Basically, the first thing to realise is that you get your number of mole of the unknown by first working out the amount of mole of the standard which was required to complete the reaction. For this you need to know the concentration of the standard, and the size of the titre that was used to complete the reaction. If the standard is in the conical flask the amount of mole will be fixed (eg. 20ml x concentration). But if the standard is in the burette the number of mole will be: titre x concentration.
So now if you have a burette full of standard that's been diluted by rinsing it with water, you're gonna need more of it to complete the reaction. That means that when you work out your concentration of unknown your gonna use:
=\frac{higher\ number \ of\ mole}{volume \ of\ pipette})
, hence you will get a higher concentration.
But then if you have the unknown in the burette that's been rinsed with water, again you're gonna get a higher titre, but the number of mole of the standard is fixed at:
)
So when you come to do your calculation you're gonna use (if it's 1 to 1):
=\tex\frac{n(standard)}{higher\ titre})
, hence you will get a lower concentration for the unknown.
Hope that helps...
edit* same rules obviously apply if it's not one to one. And similar logic applies for pippettes:
Unknown in burette:
If the pipette is rinsed with water, the volume of the titre will be smaller giving
=\frac{n}{smaller\ titre})
so you get a higher concentration.
Standard in burette:
If pipette is rinsed with water, smaller titre so,
=\frac{smaller\ n}{V\ of\ pipette})
giving a lower concentration.
Might seem complicated, but you only really need to think about if the titre is larger or smaller, and if your using the volume of the titre or the pipette to find the concentration of the unknown.
Home
Help
Search
Login
