The percentage of silver impurity in a lead sample was determined by the following procedure: A sample of lead weighing 1.050 g was dissolved in nitric acid to produce an aqueous solution of Pb2+ and Ag+. The solution was made up to a volume of 350.0 ml with water; a pure silver electrode was immersed in the solution and the potential difference between this electrode and a standard hydrogen electrode was found to be 0.503 volts. What was the percent silver impurity in the lead metal?