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### DENSITY

Presented by Dr / Osama Saher

Density

Definitions & Laws

Specific gravity

Specific volume

Exp.I. Determination of the density of a given liquid at a definite temperature using pycnometer

Experiments

Exp.II. Determination of the density of solutions containing different concentrations of sodium chloride at room temperature

It is the mass per unit volume at a definite (Temp.) and (Pressure)

Units of Density:

1- In Metric (CGS) System:g/cm3 (g/ml)

N.B.:

- 1 ml or 1 cm3 of water weighs 1 g at 4Co
- 1 g is defined as the mass of 1 cm3 of water at 4oC (i.e Density of water = 1 g/ml at 4 co )

2- In English System: gr/fl ℥ - lb/gal

Density (ρ) = Mass/Volume

It is the ratio of the weight of a substance to the weight of an equal volume of another substance chosen as a standard. Both substances having the same temperature.

Water Standard for liquids and solids

Hydrogen / Air Standard for gases

Specific gravity =Ws / Ww = ρs /ρw

Density versus Specific Gravity

Density

- Concrete number (has unit)
- Vary in value with the systems of measure

e.g. for water: 1 g/ml, 455 gr/fl ℥ , 62.5 lb/ft3

Specific Gravity

- Abstract number (no unit), being a ratio
- Constant value (at constant temp.)

e.g. for water: Sp. gravity = 1

- N.B.:
- Density = Specific Gravity (for value) in Metric system only not in English system
- Standard temp. used in Sp. Gravity = 25oC for all substances, except alcohols (15.56oC)

- It is expressed in terms of liter, ml or cubic centimeter (cm3).
- The volume of l g of water at 1 atmospheric pressure and 4°C was equivalent to 1 cm3. (One liter = 1000.027 cm3).

It is the volume occupied by 1 g of a substance at a given temp. and pressure(reciprocal of density)

Specific volume

V = 1/ρ (cm3/g)

Exp.I. Determination of the density of a given liquid at a definite temperature using pycnometer

- Specific gravity bottle (pycnometer) is made of glass, with 10, 25 and 100 ml capacity.
- A capillary stopper is fitted into its mouth to allow the removal of excess liquid and obtain accurate volume.

Capillary tube

Procedure:

1. Clean the pycnometer by rinsing with water.

2. Weigh the stoppered dry pycnometer.

3. Fill the pycnometer with purified water and place the stopper.(Small amount of water comes out the capillary, wipe out the excess liquid outside the tube using tissue paper).

4. Weigh the pycnometer with water.

5. After removing the water, rinse the pycnometer with the test liquid (5 ml) and fill the liquid in the pycnometer in the same way.

6. Weigh the pycnometer with the test liquid.

7. Repeat each step at least three times and report the average.

Observations:

- Volume of pycnometer (V)
- Weight of empty, dry pycnometer with stopper (W1)
- Weight of pycnometer with water (W2)
- Weight of pycnometer with test liquid (W3)

Calculations:

- Weight of water in pycnometer (W2 - W1)
- Weight of test liquid in pycnometer (W3 - W1)
- Density of liquid

accurate volume of pyc. is known

Weight of liquid (W3 – W1)

Ρ(liq) = ——————————

Volume of liquid (V)

accurate volume of pyc. is unknown

Weight of liquid (W3 – W1)

Ρ(liq) = —————————— X Ρ(water)

Weight of water (W2 – W1)

Sp. Gravity

A specific gravity bottle weighs 23.66 g. When filled with water it weighs 72.95 g; when filled with another liquid it weighs 73.56 g. What is the specific gravity of the liquid?

Solution:

73.56 g - 23.66 g = 49.90 g of liquid

72.95 g - 23.66 g = 49.29 g of water

- Specific gravity of liquid = 49.90 (g)/49.29 (g) = 1.012
- Density of liq. =

Specific gravity of liq. X Density of water

= 1.012 g/cm3

Exp.II. Determination of the density of solutions containing different concentrations of sodium chloride at room temperature

1% w/v

2% w/v

4% w/v

10% w/v

Unknown

- To study the effect of added salt on density
- To determine the concentration of unknown solution after determination of its density.

1. Follow the procedure in experiment no.1.

2. Determine the density of the different sodium chloride solutions (1-10%) as described before.

3. Plot a graph between the concentration of sodium chloride solutions and density. Comment on the plot concerning the effect of concentration on density.

4. On the basis of the graph, determine the concentration of an unknown sodium chloride solution after determining its density.

Observations:

1. Temperature (room temperature) = …… °C

2. Weight of pycnometer at room temperature (W1) = …… g

3. Weight of pycnometer + water (W2) = …… g 4. Weight of pycnometer + solutions of different concentrations of NaCl in water = (Wc).

(Use tableA to record your data).

5. Calculate the density of different NaCl solutions. Tabulate your results in table B.

Results:

- Increasing conc. of NaCl will increase the density of the solution.
- From the graph, we can determine the conc. of unknown solution of NaCl after determination of its density.

Density

(g/ml)

Intercept at value 1

(water density)

Nacl Conc.

(%w/v)

What is drawn on Y-axis (dependant variable) and on X-axis (independant variable)

Shape of resulting curve

Slope and any special points

What to be concluded from this shape, slope, special points

So, Let’s apply this on our curve

PRECAUTIONS

- Clean and dry Pycnometer.
- Same Pycnometer for both water and liquid, if broken????????.
- Capillary Stopper should be fitted inside the Pycnometer.
- The Capillary should be directed upwards, not downwards.
- Drain the excess coming out the capillary with a tissue paper.
- No air bubbles inside the capillary or pyc. (if any, refill and close pycnometer again and drain the excess).

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