Calculate the pH of a 0.5 M Formic Acid (HCOOH) soln. ka= 0.00018
Ka=[H][COOH]/([HCOOH]
ka= x^2/(.5-x)
solve this for x
x^2+.00018x-.5*.00018=0
x=(-.00018+-sqrt(.00018^2+.00036))/2
x=-.00009+- .00948= .00939 check all that math.
pH=-log (.00939)=2.03
check the math.
The equilibrium is given by
HCOOH -> H+ + HCOO-
so Ka=[H+][HCOO-]/[HCOOH]
if we start with 0.5M HCOOH then at equilibrium there is [H+] = x and [HCOO-]=x and HCOOH=0.5-x
so Ka = (x)(x)/(0.5-x)=0.00018
this gives a quadratic to solve, however, we only want the answer to 1 sig fig (.5M is 1 sig fig) so we can say that 0.5-x is approximately equal to 0.5, because x will be small.
thus x^2/0.5=0.00018
hence you can find x
pH is then -log(x)
which is pH=2, but check the maths.
lasaga
To calculate the pH of a formic acid solution, you need to use the given value of the acid dissociation constant (Ka) and the concentration of the acid (HCOOH).
Formic acid (HCOOH) can dissociate in water to produce hydrogen ions (H+) and formate ions (HCOO-). The balanced chemical equation for this dissociation is:
HCOOH β H+ + HCOO-
The Ka value represents the equilibrium constant for this dissociation reaction. It is given as 0.00018.
The Ka expression is given by:
Ka = [H+][HCOO-] / [HCOOH]
Since we are given the concentration of formic acid ([HCOOH]), which is 0.5 M, we can assign this value to [HCOOH] in the equation.
0.00018 = [H+][HCOO-] / 0.5
To determine the concentration of hydrogen ions ([H+]), we need to assume that the concentration of formate ions ([HCOO-]) will be equal to the concentration of hydrogen ions formed ([H+]) at equilibrium. This assumption is valid because formic acid is monoprotic, meaning it produces only one hydrogen ion when it dissociates.
Therefore, the concentration of formate ions ([HCOO-]) is also 0.5 M.
0.00018 = [H+][0.5] / 0.5
Simplifying the equation, we find:
0.00018 = [H+]
The concentration of hydrogen ions ([H+]) is 0.00018 M.
The pH of a solution is given by the negative logarithm (base 10) of the hydrogen ion concentration:
pH = -log[H+]
Therefore, to calculate the pH, we substitute the value of [H+] into the equation:
pH = -log(0.00018)
Using a calculator, the pH of the 0.5 M formic acid solution is approximately 3.74.