Question

What do you understand by standard temperature and pressure?

Answer 1

Some textbooks define it differently than others, but the newest IUPAC standard temperature and pressure are:

$T_{STP}=0^{o}C=273.15K$

$P_{STP}=1bar$.

Some older ( and potentially fairly recent ) textbooks might say:

$T_{STP}=0^{o}C=273.15K$

$P_{STP}=1atm$

The difference between the two pressures are subtle but significant:

$1bar=1.00000\times {10}^{5}Pa$

$1atm=1.01325\times {10}^{5}Pa$

This leads to a difference about $0.3L$ for the molar volume $(\overline{V}=\frac{V}{n})$ of an ideal gas at STP when you calculate it using the Ideal Gas Law: $\sim 22.7L$ for the former, and $\sim 22.4L$ for the latter.