DENSITY Presented by Dr / Doaa hegazy

Items to be discussed today: Exp.I. Determination of the density of a given liquid at a definite temperature using pycnometer Exp.II. Determination of the density of solutions containing different concentrations of sodium chloride at room temperature Density Specific volume Specific gravity Experiments Definitions & Laws

It is the mass per unit volume at a definite (Temp.) and (Pressure) It is the mass per unit volume at a definite (Temp.) and (Pressure) Units of Density: 1- In Metric (CGS) System: g/cm 3 (g/ml) N.B.:  1 ml or 1 cm 3 of water weighs 1 g at 4C o  1 g is defined as the mass of 1 cm 3 of water at 4 o C (i.e Density of water = 1 g/ml at 4 c o ) 2- In English System: gr/fl ℥ - lb/gal Density ( ρ ) = Mass/Volume Density

If 250 mL of alcohol weighs 203g, what is its density? Density (  ) = mass/volume Density (  ) = 203/250= g/mL

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 (Relative density) = Specific gravity (Relative density) = ρ s / ρ w = W s / V s W w / V w W w / V w = W s / W w Specific gravity

weight of 150 mL of water= 150 g Specific gravity =W s / W w Specific gravity= 170 g/150 g= If 150 mL of polyethylene glycol 400 weigh 170 g, what is its specific gravity?

Density versus Specific Gravity Density 1. Concrete number (has unit) 2. Vary in value with the systems of measure e.g. for water: 1 g/ml, 455 gr/fl ℥, 62.5 lb/ft 3 Specific Gravity 1. Abstract number (no unit), being a ratio 2. 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 systemDensity = Specific Gravity (for value) in Metric system only not in English system Standard temp. used in Sp. Gravity = 25 o C for all substances, except alcohols (15.56 o C)Standard temp. used in Sp. Gravity = 25 o C for all substances, except alcohols (15.56 o C)

It is expressed in terms of liter, ml or cubic centimeter (cm 3 ). It is expressed in terms of liter, ml or cubic centimeter (cm 3 ). The volume of l g of water at 1 atmospheric pressure and 4°C was equivalent to 1 cm 3. (One liter = cm 3 ). The volume of l g of water at 1 atmospheric pressure and 4°C was equivalent to 1 cm 3. (One liter = cm 3 ). It is the volume occupied by 1 g of a substance at a given temp. and pressure (reciprocal of density) V = 1/ ρ (cm 3 /g) Specific volume Volume

If 120 g of acetone measures 150 mL, what is its specific volume? V= 150 mL/120 g= 1.25 mL/g V= 150 mL/120 g= 1.25 mL/g V = 1/ ρ (cm 3 /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 three times and report the average.

Empty pycnometer (W 1 ) Water filled pycnometer (W 2 ) Liquid filled pycnometer (rinse with 5ml first) (W 3 )

Observations: 1. Volume of pycnometer (V) 2. Weight of empty, dry pycnometer with stopper (W 1 ) 3. Weight of pycnometer with water (W 2 ) 4. Weight of pycnometer with test liquid (W 3 ) Calculations: 1. Weight of water in pycnometer (W 2 - W 1 ) 2. Weight of test liquid in pycnometer (W 3 - W 1 ) 3. Density of liquid

Density of liquid accurate volume of pyc. is known Weight of liquid (W 3 – W 1 ) Ρ (liq) = —————————— Volume of liquid (V) accurate volume of pyc. is unknown Weight of liquid (W 3 – W 1 ) Ρ (liq) = —————————— X Ρ (water) Weight of water (W 2 – W 1 ) Sp. Gravity

Example: A specific gravity bottle weighs g. When filled with water it weighs g; when filled with another liquid it weighs g. What is the specific gravity of the liquid? Solution: g g = g of liquid g g = g of water  Specific gravity of liquid = (g)/49.29 (g) =  Density of liq. = Specific gravity of liq. X Density of water = g/cm 3

Exp.II. Determination of the density of solutions containing different concentrations of sodium chloride at room temperature 1% w/v2% w/v4% w/v10% w/v Unknown 1)To study the effect of added salt on density 2)To determine the concentration of unknown solution after determination of its density.

Procedure: 1. Follow the procedure in experiment no 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 1. Temperature (room temperature) = …… °C 2. Weight of pycnometer at room temperature (W 1 ) = …… g 2. Weight of pycnometer at room temperature (W 1 ) = …… g 3. Weight of pycnometer + water (W 2 ) = …… g 4. Weight of pycnometer + solutions of different concentrations of NaCl in water = (W c ). 3. Weight of pycnometer + water (W 2 ) = …… g 4. Weight of pycnometer + solutions of different concentrations of NaCl in water = (W c ). (Use table A to record your data). (Use table A to record your data). 5. Calculate the density of different NaCl solutions. Tabulate your results in table B. 5. Calculate the density of different NaCl solutions. Tabulate your results in table B.

Results: Table(A) Replicates Weight (W c ) (g) Concentration of sodium chloride in water % w/v unknown a b c

Results: Table(B) Sample. Number Conc. of NaCI in water (% w/v) Weight of Solutions (W s ). Average weight (a+b+c) /3 Density (Calcula ted) (Ws / Ww) x (Ws / Ww) x ρ w Replicates abc unknown

General Comment on Graphs 1.What is drawn on Y-axis (dependant variable) and on X-axis (independant variable) 2.Shape of resulting curve 3.Slope and any special points 4.What to be concluded from this shape, slope, special points So, Let’s apply this on our curve

Results: Increasing conc. of NaCl will increase the density of the solution. 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. From the graph, we can determine the conc. of unknown solution of NaCl after determination of its density. Density (g/ml) Nacl Conc. (%w/v) Intercept at value 1 (water density)

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).