posted by Zekariyas .
How many moles of NH4Cl must be added to 1.5L of 0.2 M solution of NH3 to form a buffer whose PH is 9.(Kb=1.8x10^-5)
Use the Henderson-Hasselbalch equation.
pH = pKa + log[(base/(acid)]
You know pH, you can calculate pKa (convert Kb to pKb then pKa + pKb = pKw = 14), you know (NH3), solve for (acid; i.e., NH4Cl). Then M NH4Cl = mols/L. You know M and L, solve for mols. Then mols = grams/molar mass. You know mols and molar mass, solve for grams.
Post your work if you get stuck.