Question

How do you determine the molar mass of the unknown metal?

Data:

HCl=10mL

initial mass of metal=.31g

final mass of metal=.20g

initial pH=1.43

final pH=3.88

100 mL of water added.

Answer 1

HCl = 10 mL

initial mass of metal = 0.31g

final mass of metal = 0.2g

remaining mass of metal = 0.11g

initial pH = 1.43

-log[H+] = 1.43

[H+] = 10^-1.43 = 0.0371

number of moles of [H+] = 0.0371*10 = 0.371 mmol

final pH = 3.88

-log[H+] = 3.88

[H+] = 10^-3.88 = 1.318*10^-4

number of moles of [H+] = 1.318*10^-4*(100+10) (since there is added 100 mL of water to 10 mL of solution)

                                       = 0.0145 mmol

number of moles [H+] consumed = 0.371 - 0.0145 = 0.3565 mmol

M + 2HCl -----> MCl2 + H2

2 moles of acid require 1 mole of Metal

0.3565 mmoles of acid requires 1/2*0.3565 mmoles of metal

                            = 0.1782 mmole of metal

number of moles of Metal = mass of the metal /molar mass of the metal

0.1782*10^-3moles = 0.11g/M

M = molar mass of the metal = 0.11/(0.1782*10^-3) = 617.28 g/mol