In the density determination of a liquid, it was necessary to use the volumetric
ID: 1003190 • Letter: I
Question
In the density determination of a liquid, it was necessary to use the volumetric pipet properly. A student needed to deliver exactly 50.0 mL of a liquid. How will the quantity of liquid be affected by the situations described below, and how will the density determination be affected? A dirty pipet is used and droplets of liquid adhered to the inner walls of the pipet. The student did not allow sufficient time for all the liquid to empty from the pipet. The student allowed all the liquid to drain and then blew out the small amount from the tip. Air bubbles were not removed from the pipet before delivering the liquid. At the mark on the pipet, the student read the upper edge of the meniscus and not the lowest point on the curve.Explanation / Answer
a) Using a dirty pipet causes too little or contaminated solution to be delivered. Leaving little droplets behind on the walls (except for the small amount in the tip) causes too little solution to be delivered.This decreases the liquid volume thus increasing the density than the actual density.
b)Similar to above volume will become less thus density will be high.(density = mass/volume)
c)If so, they are designed to dispense a certain volume of liquid by gravity and there will always be a little left in the tip. If this is blown out then the volume dispensed will be greater than the volume indicated by the pipet.
For example, say we are dispensing 50mL of water and weighing it on a balance to calculate density.
Density = mass/volume
If we blew the extra out then the mass would be larger than it was supposed to be and the density calculation would be larger.
d) If there are bubbles of air then we will have less liquid than we are intending to have. Assuming we are calculating density via measurementof mass then the mass we get will be less than it should be for that volume of that liquid. As a result the calculated density will work out lower than it should do.
e)Lower surface is close to surface of container and it gives better reading compared to upper meniscus.