Which contain more atoms 1g of H2 or 20 g of CO2?

Let us see.

1 g H2 (1 mole H2/2.016 grams)(6.022 X 10^23/1 mole H2)

= 3.0 X 10^23 atoms hydrogen gas
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20 g CO2 (1 mole CO2/44.01 grams)(6.022 X 10^23/1 mole CO2)

= 2.74 X 10^23 atoms of carbon dioxide

By a marginal amount hydrogen gas contains more atoms at 1 gram than CO2 at 2o grams.