Question

Conductivity of Aqueous Solutions: Electrical conductivity is based on the flow of ions. Slightly ionized substances are weak electrolytes. Weak acids and bases would be categorized as weak electrolytes because they do not completely dissociate in solution. Highly ionized substances are strong electrolytes. Strong acids and salts are strong electrolytes because they completely ionize in solution. The ions carry the electric charge through the solution, thus creating an electric current. The current, if sufficient enough, will light one or both LEDs on a conductivity meter, shown at right. The meter has a 9V battery, two parallel copper electrodes and two LEDs, one green and one red. The conductivity of a solution can be tested by switching the meter on and dipping the copper electrodes in the solution to be tested. Substances that do not conduct an electric current are called non-electrolytes. Non-electrolytes do not ionize; they do not contain moveable ions. The LEDs of a conductivity meter will not light because there are no ions to carry the electric current. The table given below is a guide to the possible conductivity measurements. Scale Red LED Green LED Conductivity 0 off off low or none 1 dim off low 2 medium off medium 3 bright dim high 4 very bright medium very high

Based on the information provided above, answer the following questions:

I. Is it possible to identify whether the given solution is 1 M NaOH or 1 M HCl using the conductivity meter? Justify your answer. (1)

II. What is the possible pH value of the solution if the glow of the green LED is medium and the red LED glows very brightly? (1)

(i) 1 (ii) 13 (iii) 5 (iv) 8

1. (i) and (ii)
2. (i) and (iii)
3. (ii) and (iv)
4. (iii) and (iv)

OR

Write down the observations if the conductivity meter is dipped in distilled water.

III. Refer to the table given below and draw the molar conductivity vs. concentration curve for solutions A and B. (2)

Solution	Red LED	Green LED
A	bright	dim
B	dim	off

Answer 1

I. No. Both have strong electrolytes, so both LEDs will glow. In both cases, red will be very bright, and green will be medium.

II. (i) and (ii)
OR

Scale	Red LED	Green LED	Conductivity
0	off	off	low or none

III. A is a strong electrolyte, while B is a weak electrolyte.