Question

When 0.1 mol of ammonia (NH₃) is dissolved in sufficient water to make 1 L of solution, the solution is found to have a hydroxide ion (OH⁻) concentration of 1.5×10⁻³ M . The value of Kb for NH₃ is:

A. 8.65×10⁻² M
B. 0.5×10⁻³ M
C. 2.28×10⁻⁵ M
D. 5.75×10⁻⁶ M

Answer 1

To calculate the Kb for NH₃, use the expression Kb = [NH₄⁺][OH⁻] / [NH₃]. With the given OH⁻ concentration, Kb is approximately 2.25×10⁻µ M, so the closest provided option is 2.28×10⁻µ M (option C). Therefore, the correct option is C.

Step-by-step explanation:

When 0.1 mol of ammonia (NH₃) is dissolved in sufficient water to make 1 L of solution, the resulting hydroxide ion (OH⁻) concentration is given as 1.5×10⁻³ M. To find the Kb for NH₃, we can use the formula for the base ionization constant, which is Kb = [NH₄⁺][OH⁻] / [NH₃]. Since we have the concentration of OH⁻ and the initial concentration of NH₃, we can assume that the change in concentration of NH₃ is roughly equal to the concentration of OH⁻ because NH₃ is a weak base and doesn't dissociate completely.

We can set up the expression as follows: Kb = (1.5×10⁻³)^2 / (0.1 - 1.5×10⁻³). Plugging in the values, we find that Kb is approximately 2.25×10⁻µ M. From the options provided, the closest value to our calculated Kb is 2.28×10⁻µ M.

Therefore, the correct answer is C. 2.28×10⁻µ M.