A 0.185 M solution of a weak acid (HA) has a pH of 2.95. Calculate the acid ionization constant (Ka) for the acid.

Video transcript

Hello everyone. Today, we have the following problem. A 0.185 molar solution of weak acid has a ph of 2.95. Calculate the acid ionization constant for the acid. So our first step is to create an I CE table. So we have our acid reacting with water in a reversible reaction to form our conjugate base. In our conjugate acid. We have our I ce the I stands for our initial concentrations. So for our ain, we have an initial concentration of 0.185 molar water is not included in this because liquids and solids are not included. And then we have our concentrations of our conjugate base and our acid which are zero. We didn't have the changes that would occur because we would have minus X as a variable because we're taking away from our acid. And we would add X to our product side. E will stand for our, essentially, we're equaling the values of our I and our C. So we have 0.185 minus X for our reactant. And then we have plus X and then plus X for our products, we then need to calculate our acid or the concentration of acid. And we can do so with the equation of 10 race, the power of negative P, we were given a ph of 2.95. So we raised that negative 2.95 to the power of 10. And we get a concentration of 0.00112 molar. The last step is to calculate for our K A. And that would simply be the concentration of our conjugate base multiply by the concentration of our acid or conjugate acid divided by the concentration of our initial acid. So we have the concentration of our conjugate acid which would be 0.00112 molar and we have that twice. So we can square that. And the reason why that is such is because that was our X value. And we said that X was our variable for our conjugate base and acid. So then we divide that by the concentration of our initial acid which was 0.185 minus X and X was 0.0012 molar. And when we solve for our K, we get 6.82 times 10 to the negative six or answer choice B overall I hope is helped. And until next time.