Question

How many mL of 12 M HCl will be needed to prepare 500 ml of 1.2 ppm HCl solution? What is the pH of this 1.2 ppm HCl solution? Using the right significant figures. MW of HCl=36.5g/mol

I already found the first answer which is 1.4 x 10^-3 mL

How do I find the pH? I know the formula is -log [H+], but I have no information for it?

Answer 1

It is very simple. You have prepared 500 mL of 1.2 ppm HCl solution. For this purpose you have taken stock solution and calculated how much stock solution has to be taken and dilute to get desired concentrated solution. Forget about stock solution now

come back to 500 mL of 1.2 ppm HCl solution. 1 ppm = 1mg/ml; 500 mL contains 500 mg = 0.5 g

molarity = moles/liter;

moles = weight of HCl / molar mass of HCl

molarity = (weight of HCl in g / molar mass of HCl in g/mol) * (1000 / volume in mL)         

          = (0.5 g / 36.5 g/mol) x (1000 / 500 mL)

          = 0.0274 M

[H+] = 0.0274                    [ molarity is an unit for expressing concentration of H+ ions also]

pH = -log[H+] = -log[0.0274] = 1.56

pH = 1.56