If the balloon had expanded more how would the mass of h2 and the mass percent change?.

The mass of the balloon decreases as the release of hydrogen gas.

If the balloon had expanded more then the mass of hydrogen gas will increase inside the balloon but the mass of the balloon decreasing because the hydrogen gas escape from the system means from the balloon.

There is less percent change in the mass of the balloon while on the other hand, the concentration of hydrogen gas increases inside the balloon which leads to expansion of balloon so we can conclude that the mass of the balloon decreases as the release of hydrogen gas.

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3 hours because it is a decent distance.