At standard temperature and pressure, a given sample of water vapor occupies a volume of 2.8 0 L. How many moles of water vapor are present?

The idea here is that an ideal gas kept under Standard Temperature and Pressure (STP) conditions has a molar volume equal to #"22.7 L"#.

In other words, when pressure is set to #"100 kPa"# and temperature to #0^@"C"#, one mole of any ideal gas occupies #"22.7 L"# - this is known as the molar volume of a gas at STP.

So, if your sample of water vapor occupies #"2.80 L"# at STP, it follows that it must contain

#2.80 color(red)(cancel(color(black)("L"))) * overbrace("1 mole"/(22.7color(red)(cancel(color(black)("L")))))^(color(purple)("molar volume of a gas at STP")) = "0.12335 moles"#

of water vapor. You need to round this off to three sig figs, the number of sig figs you have for the volume of water vapor

#"no. of moles of water vapor" = color(green)(|bar(ul(color(white)(a/a)"0.123 moles"color(white)(a/a)|)))#

It's worth noting that many textbooks and online sources still list STP conditions as a pressure of #"1 atm"# and a temperature of #0^@"C"#.

Under these conditions, one mole of any ideal gas occupies #"22.4 L"#. If this is the value given to you for the molar volume of a gas at STP, simply redo the calculation using #"22.4 L"# instead of #"22.7 L"#.