The empirical formula of lead (II) oxide was determined by passing excess dry hydrogen gas over 6.69

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Question:

The empirical formula of lead (II) oxide was determined by passing excess dry hydrogen gas over 6.69 g of heated lead (II) oxide.
a). What was the purpose of using excess dry hydrogen gas?
b). The mass of the lead was found to be 6.21 g. Determine the empirical formula of the oxide. (Pb = 207.0; 0 = 16.0).