The question is: Calculate the pH of a solution that results from 50.0 ml of 0.15mol/L HCOOH with 75.0ml of 0.13mol/L HCOONa.

I know I need to use the henderson-hasselbalch equation with the pka being 3.74 but I'm not sure how to get the ratio to plug in.

To calculate the pH of the solution using the Henderson-Hasselbalch equation, you need the ratio of the concentration of the conjugate base (HCOO-) to the concentration of the acid (HCOOH). Here's how you can find that ratio:

Step 1: Determine the number of moles of HCOOH and HCOONa.

The number of moles can be calculated using the formula:
moles = concentration (mol/L) x volume (L)

For HCOOH:
moles of HCOOH = 0.15 mol/L x (50.0 mL / 1000 mL/L)
moles of HCOOH = 0.0075 mol

For HCOONa:
moles of HCOONa = 0.13 mol/L x (75.0 mL / 1000 mL/L)
moles of HCOONa = 0.00975 mol

Step 2: Use the balanced chemical equation to determine the mole ratio of HCOO- to HCOOH.

The balanced chemical equation for the dissociation of HCOOH is:
HCOOH ⇌ HCOO- + H+

From the equation, you can see that one mole of HCOOH produces one mole of HCOO-.

Step 3: Calculate the ratio of HCOO- to HCOOH.

ratio of HCOO- to HCOOH = moles of HCOONa / moles of HCOOH
ratio of HCOO- to HCOOH = 0.00975 mol / 0.0075 mol
ratio of HCOO- to HCOOH ≈ 1.3

Now that you have the ratio of the conjugate base to the acid, you can plug it into the Henderson-Hasselbalch equation to calculate the pH of the solution.

pH = pKa + log(ratio of HCOO- to HCOOH)
pH = 3.74 + log(1.3)

Using a calculator or logarithmic tables, you can evaluate the logarithm to find the pH of the solution.

Note: The Henderson-Hasselbalch equation assumes that the acid and its conjugate base are in equilibrium. Make sure the concentrations of the acid and its conjugate base are given at the same temperature to use this equation accurately.