How many moles are contained in 78.41 L of Ne at STP?

Hint: Use the ideal gas equation, and substitute the values given in the question, in order to get the number of moles. In STP conditions, the temperature and pressure are known, so in order to find the number of moles, we will use the equation $PV=nRT$.

Complete step-by-step answer: In order to answer the question, we need to know about moles and molar mass. Now, matter is made up of atoms, and as matter has mass, then the atoms should have an individual mass. Molar mass of an element or compound is the mass which houses $6\times {{10}^{23}}$ particles. For, example, the hydrogen molecule has a molar mass of 2 grams. This means 2 grams of hydrogen contains $6\times {{10}^{23}}$atoms, and this number is also called the Avogadro’s number.
Number of moles of an element or a compound is the ratio of its given mass taken by the user, to its molar mass. More is the number of moles, more is the concentration of the substance. Now, let us come to the question.
STP stands for standard temperature and pressure conditions and in STP, 78.41 litres of neon as corresponds to 3.498 moles of the gas. Now, we will use the ideal gas equation to solve for the number of moles. In STP conditions, pressure is taken to be 1 atmosphere, temperature is taken to be 273K and R is the universal gas constant, which is $0.0821\,L\,atm\,mo{{l}^{-1}}K$. So, the number of moles can be calculated in the following way:

\[
   PV=nRT \\
 \Rightarrow n=\dfrac{PV}{RT} \\
  \Rightarrow n=\left( \dfrac{1\times 78.41}{0.0821\times 273} \right) \\
  \Rightarrow n=3.498mole \\
\]
So, we get the moles of the neon gas at STP to be 3.498 mole, which is the required answer for the question.

Note: It is to be noted that contrary to STP conditions, there is also an NTP condition, which refers to normal temperature and pressure. Temperature is taken as ${{20}^{0}}C$ and pressure is taken to be 1 atm.