Density Flashcards
Define mass density
Mass density is defined as mass per unit volume.
Define relative density
Also known as specific gravity. It is the ratio of density of a substance to a density of a reference material.
Relative density of liquids
Density of sample over density of water.
Density of water at 4 deg C
<1.00 g/mL but rounded off to 1.00 g/mL
Variation of density with temperature.
Density decreases with increasing temperature
Variation of density with pressure.
Density increases with increasing pressure.
Variation in density are more apparent in ___, because? State the formula
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
T/F: Density increases with molar mass
True
Usual range of densities of solid
1 - 20 g/mL
Usual range of densities of liquid
1 - 2 g/mL
Usual range of densities of gas
«< 1 g/mL
Rank according to increasing densities: solid, liquid, gas
gas «< liquid < solid
Gravimetric method of determining solid density
Displacement method. V Water displaced = final - initial volume
Gravimetric method of density determination of liquids
Using pycnometer that is calibrated against reference liquid.
Archimedes principle
an object partially, or fully immersed in a fluid experiences a buoyant force acting upon it
T/F: Fluid density decreases as buoyant force increases. State the formula of buoyant force.
False, Fb = pgV
Four density determination method
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
In hydrometer, d>1.0 g/mL if the instrument floats or submerged in reference with the line?
Floats
What is the density of liquid if the hydrometer submerged in reference to the line?
d < 1.0 g/mL
T/F: In oscillating densitometer, the higher the mass of the sample, the lower the frequency
True
How temperature of objects affects the mass of the object in gravimetric method?
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.
T/F: There is a major change in density because of pressure.
False, the change in air buoyancy due to pressure is negligible
How purity affects the determination of density?
If a contaminant (occluded space, air, etc) has lower density than object, measured density is lower
How viscosity affects the determination of density?
Viscosity lessens the frequency of vibration while using oscillating densitometer
Give the applications of density determination.
Characterization
Determination of concentration of binary mixtures (salt, sugar, alcohol solutions)
Quality control
How to perform initial cleaning of pycnometer?
Rinse with tap water with little DL
Rinse with tap water again
Rinsed with DW 3x
Wipe outside with lint-free cloth
How to dry the inside of pycnometer?
Rinse with small amount of acetone or 95% ethanol then diethyl ether
Dry using a blower under fume hood
How to find the volume of pycnometer?
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
Formula for volume of pycnometer
(m w/ water - m w/o water)/density at temp of water
Formula for density of liquid sample
(m w/ sample - m w/o sample)/V of pyvnometer
Concentration and density relationship of alcohol-water solution. Explain why.
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.
