STP is the short way of saying "Standard Temperature and Pressure". It describes the set of conditions usually used when describing the volume of a gas. "Standard Temperature" is agreed upon to be either 0°C or 273 K, while "Standard Pressure" means 1 atmosphere, 760 torr, or 101.3 kPa. Under STP conditions, one mole of any gas occupies 22.4 L of volume. That volume will be different at any other pressure or temperature. I assume that both questions are about gases at STP.
"the number of grams of neon contained in 12.3 L of Ne gas"
We can use dimensional analysis to convert 12.3 L of Ne to moles of Ne, to grams of Ne. We shall need the following equalities to construct conversion factors:
1 mol Ne = 22.4 L Ne
1 mol Ne = 20.180 g Ne
[(12.3 L Ne)/1][(1 mol Ne)/(22.4 L Ne)][(20.180 g Ne)/(1 mol Ne)] = 11.0809 g Ne or 11.1 g Ne rounded to three significant figures
Answer: The number of grams of neon contained in 12.3 L of Ne gas is 11.1 grams.
"the number of moles of H2 in 1800 mL of H2 gas"
We can use dimensional analysis to convert 1,800 mL of H2 to liters of H2, to moles of H2. We shall need the following equalities to construct conversion factors:
1 L H2 = 1,000 mL H2
1 mol H2 = 22.4 L H2
[(1,800 mL H2 )/1][(1 L H2 )/(1,000 mL H2)][(1 mol H2)/(22.4 L H2)] = 0.0803571 mol H2 or 0.080 mol H2 rounded to two significant figures
Answer: The number of moles of H2 gas contained in 1,800 mL of H2 gas is 0.080 mole.