What did lavoisier do at the beginning and end of every experiment that helped him prove his theory

Why does his teacher ask him to balance the equation by including the correct coefficient

In salt water solution,the salt is the blank

Large helium-filled balloons are used to lift scientific equipment to high altitudes. what is the pressure inside such a balloon if it starts out at sea level with a temperature of 10.0ºc and rises to an altitude where its volume is twenty times the original volume and its temperature is – 50.0ºc ?