Convert particles to moles using Avogadros number.
Avogadro's constant is now a defined quantity rather than a measured one: since the 2019 redefinition of SI units, one mole contains exactly 6.02214076 x 10^23 entities by definition. Dividing a particle count by that constant gives moles, and from there molar mass converts to grams while the molar volume of an ideal gas converts to litres. The particle count is entered as a mantissa and a power of ten because the numbers involved exceed what an ordinary input field handles gracefully — 3.011 x 10^23 particles, exactly half a mole, is a typical exam quantity and would otherwise be twenty-four digits long.
Particles to moles, mass and gas volume
Particles = mantissa x 10^exponent; moles = particles / 6.02214076e23; mass = moles x molar mass; volume at STP = moles x 22.414 L
No. Since May 2019 it is fixed by definition at exactly 6.02214076 x 10^23 per mole, and the kilogram is defined from the Planck constant instead. Earlier textbooks quote it with an uncertainty; that is now historical.
Because 22.414 L is the molar volume at STP, meaning 0 C and 1 atm. At 25 C and 1 atm — room conditions — one mole occupies about 24.5 L, which is where the other figure comes from.
No. It only means anything for a substance behaving as an ideal gas. For condensed phases ignore it and use density instead.