Density

Question 1

Define mass density

Answer 1

Mass density is defined as mass per unit volume.

Question 2

Define relative density

Answer 2

Also known as specific gravity. It is the ratio of density of a substance to a density of a reference material.

Question 3

Relative density of liquids

Answer 3

Density of sample over density of water.

Question 4

Density of water at 4 deg C

Answer 4

<1.00 g/mL but rounded off to 1.00 g/mL

Question 5

Variation of density with temperature.

Answer 5

Density decreases with increasing temperature

Question 6

Variation of density with pressure.

Answer 6

Density increases with increasing pressure.

Question 7

Variation in density are more apparent in ___, because? State the formula

Answer 7

Gases since they are compressible fluid. When pressure increases, the molecules come closer and hence, higher density and vice versa.

density of gas, p = MP/RT

Question 8

T/F: Density increases with molar mass

Answer 8

True

Question 9

Usual range of densities of solid

Answer 9

1 - 20 g/mL

Question 10

Usual range of densities of liquid

Answer 10

1 - 2 g/mL

Question 11

Usual range of densities of gas

Answer 11

«< 1 g/mL

Question 12

Rank according to increasing densities: solid, liquid, gas

Answer 12

gas «< liquid < solid

Question 13

Gravimetric method of determining solid density

Answer 13

Displacement method. V Water displaced = final - initial volume

Question 14

Gravimetric method of density determination of liquids

Answer 14

Using pycnometer that is calibrated against reference liquid.

Question 15

Archimedes principle

Answer 15

an object partially, or fully immersed in a fluid experiences a buoyant force acting upon it

Question 16

T/F: Fluid density decreases as buoyant force increases. State the formula of buoyant force.

Answer 16

False, Fb = pgV

Question 17

Four density determination method

Answer 17

Gravimetic method - weighing a known volume of liquid
Hydrostatic balance - Fb = pgV
Hydrometer - p = h0/(h0+dh)
Oscillating densitometer - based on the frequency of oscillation/vibration of glass tube with sample

Question 18

In hydrometer, d>1.0 g/mL if the instrument floats or submerged in reference with the line?

Answer 18

Floats

Question 19

What is the density of liquid if the hydrometer submerged in reference to the line?

Answer 19

d < 1.0 g/mL

Question 20

T/F: In oscillating densitometer, the higher the mass of the sample, the lower the frequency

Answer 20

True

Question 21

How temperature of objects affects the mass of the object in gravimetric method?

Answer 21

Temperature of objects affects the temperature of air around it
Warm objects will have lower masses due to change in air buoyancy.
Cold objects will have higher masses due to condensation of water vapor.

Question 22

T/F: There is a major change in density because of pressure.

Answer 22

False, the change in air buoyancy due to pressure is negligible

Question 23

How purity affects the determination of density?

Answer 23

If a contaminant (occluded space, air, etc) has lower density than object, measured density is lower

Question 24

How viscosity affects the determination of density?

Answer 24

Viscosity lessens the frequency of vibration while using oscillating densitometer

Question 25

Give the applications of density determination.

Answer 25

Characterization
Determination of concentration of binary mixtures (salt, sugar, alcohol solutions)
Quality control

Question 26

How to perform initial cleaning of pycnometer?

Answer 26

Rinse with tap water with little DL
Rinse with tap water again
Rinsed with DW 3x
Wipe outside with lint-free cloth

Question 27

How to dry the inside of pycnometer?

Answer 27

Rinse with small amount of acetone or 95% ethanol then diethyl ether
Dry using a blower under fume hood

Question 28

How to find the volume of pycnometer?

Answer 28

Get the empty weight
Fill the cleaned and dried pycnometer with distilled water.
Cap it with thermometer and capillary stopper, allowing it to overflow
Wipe
Stabilized temperature
Record the density at that temperature
Weigh with water
Compute for volume

Question 29

Formula for volume of pycnometer

Answer 29

(m w/ water - m w/o water)/density at temp of water

Question 30

Formula for density of liquid sample

Answer 30

(m w/ sample - m w/o sample)/V of pyvnometer

Question 31

Concentration and density relationship of alcohol-water solution. Explain why.

Answer 31

As percentage of alcohol increases, the density decreases. It is because water (solvent) has higher density than alcohol (solute) which means that increasing the alcohol concentration, decreases the water content that shifts the density farther to 1 g/mL.