A sample of #NH_3# gas occupies 75.0 L at STP. How many molecules is this?
Your strategy for this problem will be to determine how many moles of gas you have in that volume at STP, then use Avogadro's number to find the number of molecules of ammonia would be found in that many moles.
STP, Standard Temperature and Pressure, conditions are defined as a temperature of
Under these conditions for pressure and temperature, One mole of any ideal gas occupies exactly
This means that you can use the molar volume of a gas to determine how many moles you have in
#75.0color(red)(cancel(color(black)("L"))) * "1 mole"/(22.7color(red)(cancel(color(black)("L")))) = "3.304 moles"#
Now, you know that one mole of any substance contains exactly
#3.304color(red)(cancel(color(black)("moles"))) * (6.022 * 10^(23)"molecules NH"_3)/(1color(red)(cancel(color(black)("mole")))) = color(green)(1.99 * 10^(24)"molecules NH"_3#