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() 85% le Edit View History Bookmarks People Window Help MasteringChemistry: Hwk8: Chapter 17 /session.masteringchemistry.com/myct/itemView?assignmentProblemID-96370637 a Secure https:// Hwk 8: Chapter 17 Exercise 17.136 12 of 37 > Constants I Periodic Table PartA You are asked to prepare 2.6 L of a HCN/NaCN buffer that has a pH of 9.51 and an osmotic pressure of 1.39 atm at 298 K. What masses of HCN and NaCN should you use to prepare the buffer? (Assume complete dissociation of NaCN.) Express your answers using two significant figures separated by a comma Submit RequestAnswer Provide Feedback Next > 9

Explanation / Answer

First calculate molar concentration of buffer

osmotic pressure (p) = 1.39 atm

T = 298 K

R = gas constant

M = molarity

i = Van't hoff factor

So,

p = iMRT

1.39 = 1 x M x 0.08206 x 298

M = 0.06 M

Part A)

given volume of buffer = 2.6 L

so, moles of buffer = 0.06 M x 2.6 L = 0.156 moles

using Hendersen-hasselbalck equation,

pH = pKa + log(NaCN/HCN)

buffer pH = 9.51

pKa = 9.31

so,

9.51 = 9.31 + log(NaCN/HCN)

(NaCN) = 1.6(HCN)

and,

(HCN) + (NaCN) = 0.156 moles

(HCN) + 1.6(HCN) = 0.156 moles

(HCN) = 0.156/2.6 = 0.06 moles

mass HCN = 0.06 moles x 27.025 g/mol = 1.6215 g

(NaCN) = 0.156 - 0.06 = 0.096 moles

mass NaCN = 0.096 moles x 49.01 g/mol = 4.705 g

mHCN,mNaCN = 1.62, 4.70

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