ID Information: Last Name, First Name, date, Locker #, Course/Section #, lab partner’s name
The purpose of the experiment is to study the density of unknown solutions of different concentrations.
References:
Suzanne Slayden, 2015, Chemistry 211, 212, 251 Laboratory Experiments, 6th Edition, GMU Bookstore, Johnson Center, page 35 - 45
Materials and Chemicals:
The materials required for the experiment are:
10ml volumetric flask
Electronic gram scale
A solution which is unknown
3M NaCl solution
Procedure:
The procedure of the experiment is as follows: (Suzanne, 2015)
A clean and dry 10.00 ml volumetric flask with a stopper was weighed on an electronic balance and its mass recorded.
Dilutions:
A small quantity of the 3.0 M stock solution was used in rinsing the buret. Then, the buret was filled with the 3 M stock solution and the initial volume of the solution was recorded.
The solution in the buret was added directly into the 10.00ml volumetric flask and the final volume of the solution in the buret was then recorded.
Distilled water was added until the volumetric flask was half to three-quarters full then the mixture was swirled in order to mix the contents of the flask.
Water was then added drop wise until the bottom of its meniscus touched the calibration mark on the volumetric flask. This solution was mixed well, the mass of the solution was measured, and the actual concentration of the solution in the flask determined and recorded.
Calculation the mass of a dry 10.00ml volumetric flask with stopper was carried out.
Calculation the actual concentration of the diluted solutions was carried out using the formula. Vdil.Cdil=Vconc.Cconc
The determination of densities was achieved by weighing the volumetric flask containing the salt solution prepared in step 2 above. Using the mass and volume density was calculated
The volumetric flask was rinsed using distilled water and the remaining dilute standard solutions were prepared using the method elaborated in two above. The density for each solution prepared was determined and recorded as in step 5 above.
A graph of density vs. concentration was plotted.
Observations:
During and after the experiment it was observed that the line at the volumetric flask was not at 10ml. This may have led to errors in the results obtained.
Results and Calculations:
The formula used to calculate the volumes of 3.O M stock solutions required to prepare 10.00 ml each of 2 M, 1 M and 0.5 M salt solutions was calculated using the formula
Vconc = (Vdil x Cdil) / Cconc
For the 0.5 M,
Vconc = (10 x 0.5) / 3
= 1.67 ml
For the 1 M,
Vconc = (10 x 1) / 3
= 3.33 ml
For 2 M,
Vconc = (10 x 2) / 3
= 6.67 ml
The initial mass of the volumetric flask was 17.171 g
Using the simplistic assumption that the dissolved NaCl(s) does not affect the volume of the solvent we can determine the constants m and b in the equation Molarity = m.density + b that relate to NaCl molarity to the NaCl(aq) density.
Density of water = 1 g/ml
Molar mass of NaCl = 58.5 g/mol
Based on the assumption that NaCl does not affect volume
NaCl (s) + H2O (l) = Volume of H2O
The equation provides is:
Molarity = x * density of NaCl (aq) + y
We know that,
Molarity = number of moles of solute / volume of solution = nsolute/Lsolution
Density = mass of solution / volume of solution = msolute/ Vsolution
Therefore replacing in the equation:
(nsolute/Lsolution) = m.(msolute/vsolution) + y
In this case, Lsolution = Vsolution, which we can assume is 1L
nsolute =m x msolute +y
Solving for the slope (m)
m is the slope of the graph = (nsolute1 – nsolute2) / (msolute1– msolute2)
n solute = msolute / molar mass = msolute/58.5
Therefore, we take two mass measurements of 2g (msolute1) and 1g (msolute2) in order to solve for m
For 1g nsolute = 1/58.5 = 0.0171
For 2g nsolute = 2/58.5 = 0.0341
m = (2 -1) / (0.0341 – 0.0171)
m = 0.017
0.0314 =0.017 x 2 +y
y = 0
Therefore, the equation is
Molarity = 0.017*density + 0
The experimental values of slope m and intercept b do not compare with the values derived from the simple model above. The slope from the calculation above is 0.017 while the slope from the experiment is 0.0426. This is mainly because the experiment density was plotted on the y-axis while molarity was plotted in the x-axis.
The units for the slope and intercept from the experiment are g/ml/M and g/ml respectively.
