Complete the following assignment on this sheet or separate paper (not notebook

Question

Complete the following assignment on this sheet or separate paper (not notebook paper). Your solutions should show all work presented neatly and in pencil. Make sure you clearly indicate your answers. Problem 1. How many kilowatt-hours of electricity are required to produce 6.1 kg of metallic sodium from the electrolysis of molten NaCI(s) when the applied emf is 5.2 V. Problem 2. What is the pH of a solution prepared by mixing 302 mL of 0.901 M NH3 with 291 mL of 0.105 M HCI? (the volumes are additive)

Explanation / Answer

1)

According to Faradays firat law of electrolysis,

m = (q/F)*(M/z)

m = mass of the substance deposite = 6.1 * 10^3 g

Q = Total electrical charge passed through the substance in coloumbs

F = faraday = 96485 coloumbs/mol

M = molar mass of the substance = 23 g/mol

z = charge of the ions = 1

6.1 * 10^3 = (Q/96485)*(23/1)

Q = electrical charge = 2.559 * 10^7 coloumbs

E = Q * V

E = 2.559 * 10^7 * 5.2

E = 1.331 * 10^8 KWH

2)

HCl + NH3 ------> NH4Cl

number of moles of HCl = 0.105*291 * 10^-3 = 0.0305 moles

number of moles of NH3 = 0.901 * 302 * 10^-3 = 0.2721 moles

number of moles remaining = 0.2721 - 0.0305 = 0.2416 moles

concentration of NH3 = 0.2416*1000/(291+302) = 0.4074 mol/L

pOH = 1/2(pKb - log C) since NH3 is a weak base we have to include the pKb value

pOH = 1/2(4.7 - log(0.4074))

pOH = 2.545

pH = 14 - pOH

pH = 14 - 2.545 = 11.455

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