Question

Procedure

The following procedures were done;
To create five standard brine solutions, place 100 mL of water in five 250mL beakers and prepare the following samples by adding appropriate amounts of solid NaCl and stirring:

SAMPLE
100 ppm NaCl
500 ppm NaCl
1,000 ppm NaCl
5,000 ppm NaCl
10,000 ppm NaCl

Obtain a conductivity meter from the Equipment menu. By selecting it with a left-mouse button click and clicking on the right-mouse button, set the meter to the ATC mode. (See Introduction for a detailed explanation.)

Place the meter above the beaker. Select meter and beaker. Combine them.

Record the normalized conductivity from the meter and temperature from the thermometer (temperature can also be read from the meter in RAW conductivity mode).

Remove the meter from the beaker. Clean the sensor by inserting the meter probe in a 100mL beaker filled half way with distilled water.

Repeat steps 2 through 5 in order to determine the conductivity of at 5 standard brine solutions and 1 unknown brine solution.

Observations and Results

Table 1: 100ml of water in 250ml beaker

Sample
Conductivity (µS)

100 ppm NaCl
0.24

500 ppm NaCl
1.19

1,000 ppm NaCl
2.35

5,000 ppm NaCl
11.35

10,000 ppm NaCl
22.09

Table 2: 100ml of unknown bine solution

Sample
Conductivity (µS)

Unknown Brine Solution 1
19.48

500 ppm NaCl
20.54

1,000 ppm NaCl
21.60

5,000 ppm NaCl
29.91

10,000 ppm NaCl
40.01

Include a copy of your standard curve graph.

Discussion

What were the concentrations of the three unknown brine solutions? (Samples 1U, 2U, and 3U)
Why do you have to adjust the conductivity meter to a standard temperature? (25 C)
Why did the conductivity readings increase as the concentration of NaCl increased?
You’ve probably heard of hard water. What makes water hard? How could you use a conductivity meter to determine the hardness of water samples?
Calculate the molarity of each of the standard NaCl solutions. Be sure to show your work!
Remember:
The molecular weight of NaCl is 58.44 g/mole.
1 ppm = 1 mg/1 L so 100 ppm = 100 mg/L, and so on
Molarity = moles/L

Conclusion

<In a few sentences, provide a concluding statement about the results
of your lab.>

Answer 1

Molarity of standard NaCl solutions

In 100 ml water

ppm = mg/L

moles = g/molar mass

molarity = moles/L = ppm/molar mass x L

NaCl (ppm)                          Molarity                    

      100                     100/58.44 = 1.71 mM

      500                     500/58.44 = 8.55 mM     

     1000                    1000/58.44 = 17.11 mM

     5000                    5000/58.44 = 85.56 mM

   10,000                  10,000/58.44 = 171.11 mM

Plot log(conductivity) vs concentration

Plot is shown below

Conductivity of the unknown brine solution.

100 ml of unknown brine solution gave a reading of 19.48 us

concentration of NaCl in the unknown = 500 x 19.48/20.54 = 474.20 ppm

Conductivity of solute in a solution is related to temperature. As the temperature increase the conductivity also goes up, more ions are formed. Thus it is important to adjust the conductivity meter to a standard temperature.

Conductivity is directly proporational to the number of ions present at a time in solution. As the concentration of NaCl goes up, the available ions both Na+ and Cl- in solution also goes up, which increases the conductivity measured. Thus the conductivity readings increase with increasing NaCl concentration in solution.

Hard water contains higher amounts of minerals as opposed to soft water. Higher mineral content would result in greater conductivity. This conductivity meter can be used to determine the degree of hardness, as amount of mineral present is driectly related to the conductivity reading of water.