7.75 g of gaseous HCl are dissolved in water to give a final solution of 25.8 mL .?

Question

7.75 g of gaseous HCl are dissolved in water to give a final solution of 25.8 mL .

A) What is the molarity of this solution ?

B) What is the pH of the above solution ?

Please enter a numeric answer only, no units. Thank you

Answer 1

Hey,

a) Okay, to answer your question, you first need to find the mol of HCl:

m = 7.75g
M of HCl = 36.46 g/mol

n = m/M
= (7.75g)/(36.46g/mol)
= 0.213 mol

Now sub "n" back into the question of C = n/v [ molar concentration]
v = 25.8mL = 0.0258 L
C = (0.213mol)/(0.0258L)
= 8.26 mol/L

Therefore, the molarity of this solution is about 8.26mol/L

b) pH = - log [ 8.26 mol/L]
= -0.917

Your pH is about 0.9, which is very acidic. Ignore the "-" because it was a -log, that's why it gave you a negative answer. But if you use the negative pH and try to find the hydrogen ion concentration, by doing inverse log -( - pH ) should give you a positve answer again of 8.26 mol/L.

Hope that helps. =]

Answer 2

Convert the mass of HCl into moles. Then, divide that number of moles by the volume of the solution in Liters and you have the molarity of the solution.

Since HCl is a strong acid, it ionizes completely. So, the hydronium concentration (proton concentration) is equal to the molarity of HCl in the solution. pH is just the -log of the proton concentration.