How many atoms are in a single grain of sand?

it depends on the size of the particle

Explanation:

So a grain of salt contains about: 5.85x10-5 gr/ (29.25 gr / 6.02x1023) = 1.2x1018 atoms, half of which are sodium atoms. (The other half is chlorine atoms, of course.)

the temperature in kelvin of 2.0 dm3 of an ideal gas is doubled and its pressure is increased by a factor of four. what is the final volume of the gas

conservation of mass

mass before something happens = mass afterwards