Question

After 300 seconds, half of nitrogen (IV) oxide gas has reacted. How much of the gas will react after 900 second if the reaction is first-order?

Answer 1

After first 300 seconds, half of nitrogen (IV) oxide gas has reacted.
After next 300 seconds, half of rest of nitrogen (IV) oxide gas will react.
After last 300 seconds, half of rest of nitrogen (IV) oxide gas will react.
So after 900 seconds 0.5+0.25+0.125 = 0.875 or 87.5% of nitrogen (IV) oxide gas will react.