How many grams of HCL must you dissolve in 1400\ mL to make a solution with pH=2.7\ \ ?

2 Answers
Apr 2, 2018

"0.102 g"

Explanation:

"pH" = -log["H"^+]

2.7 = -log["H"^+]

["H"^+] = 10^-2.7 ≈ "0.002 M"

"Molarity" = "Moles of solute"/"Volume of solution (in litres)"

"0.002 M" = "n"/"1.4 L"

"n = 0.0028 mol"

Mass of "HCl" = 0.0028 cancel"mol" × "36.46 g"/cancel"mol" = "0.102 g"

Apr 2, 2018

Around 102 milligrams.

Explanation:

Well, we first address the "pH" equation, which states that,

"pH"=-log[H^+]

  • [H^+] is the hydrogen ion concentration in terms of molarity

And so,

2.7=-log[H^+]

[H^+]=10^-2.7

~~2*10^-3 \ "M"

Therefore, the hydrochloric acid solution has a concentration of 2*10^-3 \ "M"=2*10^-3 \ "mol/L".

We want to make a 1400 \ "mL"=1.4 \ "L" solution, so we must use:

(2*10^-3 \ "mol")/(color(red)cancelcolor(black)"L")*1.4color(red)cancelcolor(black)"L"=2.8*10^-3 \ "mol"

Hydrochloric acid has a molar mass of 36.46 \ "g/mol". So here, we need,

2.8*10^-3color(red)cancelcolor(black)"mol"*(36.46 \ "g")/(color(red)cancelcolor(black)"mol")=0.102088 \ "g"

~~102 \ "mg"