How many grams of HCOOH are needed to make 3.7 liters of aqueous solution of pH=3.5?
Here is a very similar problem.
http://www.jiskha.com/display.cgi?id=1286408005
I found that problem, however I'm having trouble finding the amount of gram. I found x to 3.16e-4 mol/L
You want total moles of M x L.
Then moles = grams/molar mass. Solve for grams. Are you sure about 3.16 x 10^-4? Plug that back into Ka = x^2/(3.16e-4-x) and see if you get pH of 3.5. In fact, a H^+ of 3.16 x 10^-4 gives you a pH of 3.5 (what you want); however, the question doesn't ask for H^+, it asks for grams HCOOH.
x=3.6e-4 mol/L x 3.7L x 43.06g HCOOH/mol = 0.050g HCOOH.
my online homework counted this as incorrect. am I not going about it the right way?
I don't believe either number you have quoted is correct. You can plug both 3.16 x 10^-4 or 3.6 x 10^-4 (you have each number in separate responses above) in to
Ka = (x^2)/(Y-x) where the Y is either of those numbers you have quoted above as HCOOH concn. If either number is right, x should give you a pH of 3.5 and I don't think it will.
To determine the number of grams of HCOOH needed to make a certain volume of solution with a given pH, we need to consider the dissociation of the acid and the concentration of the resulting ions.
HCOOH is a weak acid and partially dissociates in water according to the equation:
HCOOH ⇌ H⁺ + HCOO⁻
The pH value gives an indication of the concentration of H⁺ ions in a solution. In this case, a lower pH value indicates a higher concentration of H⁺ ions. Since HCOOH is a weak acid, its concentration can be approximated by the concentration of the H⁺ ions.
To calculate the concentration of H⁺ ions, we can use the following equation:
pH = -log[H⁺]
We can rearrange this equation to solve for [H⁺]:
[H⁺] = 10^(-pH)
Given that the pH is 3.5, we can calculate the concentration of H⁺ ions as follows:
[H⁺] = 10^(-3.5) = 3.16 x 10^(-4) M (moles per liter)
Now, the balanced equation for the dissociation of HCOOH shows that for every mole of HCOOH, one mole of H⁺ ions is produced. Therefore, the concentration of HCOOH will be the same as the concentration of H⁺ ions.
To determine the number of moles of HCOOH needed, we must first find the number of moles using the equation:
moles = concentration x volume
moles = 3.16 x 10^(-4) M x 3.7 L
moles = 1.17 x 10^(-3) moles
The molar mass of HCOOH is 46.03 g/mol.
To find the mass of HCOOH needed, we can use the equation:
mass = moles x molar mass
mass = 1.17 x 10^(-3) moles x 46.03 g/mol
mass ≈ 0.054 g
Therefore, approximately 0.054 grams of HCOOH are needed to make 3.7 liters of an aqueous solution with a pH of 3.5.