Question

A student uses a volumetric pipet calibrated to deliver exactly 50.0mL of a liquid and then measures the mass of the liquid. how is the density determination of the liquid affected in the situations which follow? Would the density increase, decrease, or not be affected? Explain your answers

a. dirty pipet is used and water droplets adhere to the inner walls of the pipet

b. the student did not allow sufficient time for all the liquid to empty from the pipet

c. the student blew out all the liquid from the pipet, including the small amount from the tip.

d. the student did not remove air bubbles from the pipet before delivering the liquid

e. after mark on the pipet, the student didn't read the upper edge of the meniscus, not the lowest point on the Curve.

Answer 1

Density = Mass/Volume

a.

Dirty pipet contains some dust, if the dust mass is greater than that of the liquid occuping the same volume, the mass will increase. So, density will increase. But, if the dust mass is lower than that of the liquid occuping the same volume, the mass will decrease. So, density will decrease.

b.

When the student did not allow sufficient time for all the liquid to empty from the pipet, there will left some liquid in the pipet. So, exact mass will be less. If mass is less the obviosly the density will decrease.

c.

When the student blew out all the liquid from the pipet, including the small amount from the tip. Then all lquid will be transfered along with some moisture from the mouth. So, overall mass will increase (including some moisture, which is water drop). Hence density will increase.

d.

When the student did not remove air bubbles from the pipet before delivering the liquid. The air bubble will occupy some volume, which the liquid should have occupied. So, overall mass is less, since the exact volume is not occupied by the liquid. Hence the mass is decreasing so density will decrease.