Discussion
This virtual experiment involves the determination of the concentration of salt (sodium chloride, NaCl) in an aqueous solution. Concentration may be expressed many ways, and this experiment studies two systems: molarity (M) and weight/volume percent (%w/v).
1. Molarity is defined as the number of moles of solute (in this case NaCl) in a liter of solution. It is calculated with the following equation:
M = moles solute
liters solution
2. Percent concentration (w/v) is defined as the number of grams of solute (in this case NaCl) in 100. mL of solution. It is calculated with the following equation:
%(w/v) = grams solute x 100
mL solution
The experimental method involves taking a specific volume of an aqueous salt solution and slowly evaporating it to dryness. The weight which remains is that of the solute. From the weight of solute and the initial volume of solution, both molarity and percent (w/v) can be calculated.
Experimental
In this virtual experiment, three students (A, B and C) determined the salt content of three identical samples. All the data that would have been collected in an actual experiment are recorded on the attached report form. Two concentrations, molarity and %(w/v), of salt in 5.00 mL of solution were determined based on the weight the dry salt and the volume of the solution.
Calculations
The student data is entered in the table on the next page. Based on their data, you are required to enter all the calculated values in the spaces provided in the report form.
a. Calculate the weight of the salt by subtracting the tare weight of the evaporating dish from its final dry weight
b. Convert the weight of sodium chloride into moles.
c. Convert the volume of solution into liters.
d. Calculate the molarity of the solution from the moles and liters by substituting these values into the equation for M.
e. Calculate the percent sodium chloride (w/v) from the weight of sodium chloride and the volume of solution by substituting these values into the equation for %(w/v).
Experiment #8 – Concentration of a Solution
Report Form
Name__________________________ Section ___________ Date __________________
In the table below, the experimental data collected by Students A, B and C are given on lines 1, 2 and 3. Calculate the missing values for these students, and enter these values on lines 4 to 8. Be sure to use the correct number of significant figures. In the spaces marked “Set-up”, show your work to calculate the values for Student A only. Complete all the calculations and then average the results of the three trials. On the assigned date, submit to your instructor the report form (page 2) and the complete calculations for the exercises on page 3.
| DATA | |||||||
| Line# | Student A | Student B | Student C | ||||
| 1 | Weight of the evaporating dish | 46.075 | g | 51.857 | g | 48.432 | g |
| 2 | Volume of the salt solution | 5.00 | mL | 5.00 | mL | 5.00 | mL |
| 3 | Weight of the dish and residue after evaporation | 46.587 | g | 52.355 | g | 48.937 | g |
| 4 | Weight of dry sodium chloride Set-up: Line #3 – Line #1 | g | g | g | |||
| 5 | Moles of sodium chloride Set-up: Line #4 ÷ 58.5 g/mol | moles | moles | moles | |||
| 6 | Volume of solution in liters Set-up: #2 ÷ 1000 mL | L | L | L | |||
| 7 | Molarity (M) of the solution Set-up: #5 ÷ #6 | M | M | M | |||
| 8 | Percent (%w/w) of sodium chloride Set-up: (#4 ÷ #2) x 100 | % | % | % | |||
| 9 | Average (%w/w) of sodium chloride Set-up: (8A + 8B + 8C) ÷ 3 | % |
Complete the Following Exercises
- Calculate the percent (w/v) of a solution made by dissolving 23.4 g sodium chloride in water to make a solution with a volume of 250. mL
- Calculate the number of grams of NaOH needed to make 1500. mL of a 0.200 M solution.
- 5.0 mL of a potassium chloride solution is transferred to an evaporating dish and the water evaporated. The weight of KCl is 0.75 g.
a. calculate the molarity
b. calculate the percent (w/v) KCl
