Hydrogen and oxygen combine in the ratio of 1 : 8 by mass to form water. What mass of oxygen gas would be required to react completely with 3g of hydrogen gas ?

In a reaction 5.3g of Sodium carbonate reacted with 6g of ethanoic acid. The products were 2.2g of Carbon dioxide, 0.9g water and 8.2g of Sodium ethonoate. Show that these observations are in agreement with the law of conservation of mass.