what mass of solid LiOH must be added to 10 L of 135 M hydro

what mass of solid LiOH must be added to 1.0 L of 1.35 M hydrofluoric acid solution in order to create a buffer solution with pH = 2.90? (Hint: What ratio of [HF] to [F-] is neede?) For hydrofluoric acid Ka= 6.8 * 10^-4

Solution

ka=6.8*10^-4

pka=3.17

ph=2.90

moles of HF=1.35

let moles of LiOH added=x

so 2.90=3.17+log(F-/HF)

-0.27= log(x/1.35-x)

0.763(1.35-x)=x

x=0.584 moles

mass of LiOH= .584*23.95=13.98=14 g