A sealed balloon is filled with 1.00 L helium at 23 °C and 1.00 atm. The balloon rises to a point in the atmosphere where the pressure is 225 torr and the temperature is -31 °C. What is the change in volume of the balloon?

The mass has escaped into a gas which you can not see so the law stays true.

Answer; carbon monoxide (co);

It is a mixture. simply put, a compound must be pure, that is, of only one chemical structure and composition. a mixture is made of different chemical substances (i.e. elements, molecules, compounds) that are not chemically bonded to each other, but instead physically grouped together, as in the case of gasoline (a homogeneous solution).