How many grams (to the nearest 0.01 g) of NH4Cl (Mm = 53.49 g/mol) must be added to 700. mL of 1.294-M solution of NH3 in order to prepare a pH = 9.70 buffer? (pka of NH3 = 9.25)

Use the Henderson-Hasselbalch equation.

pH = pKa + log [(base)/(acid)]
You have pH, pKa, and molarity of the base. Solve for (acid) [which in this case is the NH4Cl).
Knowing (acid), calculate grams needed to make 700 mL of that concentration.

To calculate the amount of NH4Cl you need to add to prepare a buffer solution, you can use the Henderson-Hasselbalch equation:

pH = pKa + log([A-]/[HA])

In this case, NH3 is the base (A-) and NH4Cl is the conjugate acid (HA).

Given that the pKa of NH3 is 9.25 and the pH of the buffer solution you want to prepare is 9.70, we can substitute these values into the Henderson-Hasselbalch equation:

9.70 = 9.25 + log([A-]/[HA])

Rearrange the equation:

log([A-]/[HA]) = 9.70 - 9.25

log([A-]/[HA]) = 0.45

Next, we need to calculate the ratio of [A-]/[HA] using the equation:

[A-]/[HA] = 10^(0.45)

[A-]/[HA] = 3.5481

Since the volume of the solution is given in mL, we should convert it to liters by dividing by 1000:

Volume = 700 mL / 1000 = 0.7 L

Now we can calculate the moles of NH3 and NH4Cl:

Moles of NH3 = 0.7 L * 1.294 mol/L = 0.9058 mol

Moles of NH4Cl = 0.9058 mol * 3.5481 = 3.2161 mol

Finally, we can calculate the mass of NH4Cl using its molar mass:

Mass of NH4Cl = Moles of NH4Cl * Molar mass of NH4Cl

Mass of NH4Cl = 3.2161 mol * 53.49 g/mol

Mass of NH4Cl = 172.04 g

Therefore, you need to add approximately 172.04 grams of NH4Cl (to the nearest 0.01 g) to the 700 mL of 1.294-M solution of NH3 in order to prepare a pH = 9.70 buffer.