chemistry help for a level ocr

(c). The chemist prepares 1dm3 of a buffer solution by mixing 200 cm3 of 0.200 mol dm−3 HNO2 with 800 cm3 of 0.0625 mol dm−3
sodium nitrite, NaNO2.
i. Calculate the pH of the buffer solution.
Give your answer to two decimal places.
ka = 4.69 × 10−4
and the answer is 3.43 ✔

why do you work out the moles instead of using the initial concentrations. I can't remember why