Having 2 moles of HCl, and assuming that HCl is the limiting reactant, it is expected that 1 mole of water gets formed from the reaction, but you observe that only 0.95 moles actually get formed. The explanation for that is the fact that no chemical reaction occurs with 100% efficiency. That essentially means that the actual yield (0.95 moles), will always be smaller than the theoretical yield (1 mole).
The reaction yield is a measure of the efficiency of a chemical reaction in producing the desired products. The expression for the reaction yield is given by
In the example above, the reaction yield (In percentage form) would be (0.95 moles / 1mole) = 95%. The yield with respect to mass would be
\(\frac{0.95 \: \text{moles} \times \text{molar mass}}{1 \: \text{mole} \times \text{molar mass}}\)
As you can see, the molar mass cancels out, so the yield with respect to the number of moles is the same as the yield with respect to mass.
A reaction between 8g of Hydrogen gas (\(\ce{H2}\)) and excess oxygen gas (\(\ce{O2}\)) produces 64.8 g of water (\(\ce{H2O}\)). What is the percent yield?
Solution: The balanced reaction of formation of water is
\(\ce{2H2 + O2 -> 2H2O}\)
For every mole of hydrogen gas, one mole of \(\ce{H2O}\) is formed. 8g of Hydrogen gas is 8g/(2g/mol) = 4 moles of \(\ce{H2}\), so the theoretical yield of water is 4 moles, which is 72g. Dividing the actual yield by the theoretical yield gives
\(\frac{64.8 \: g}{72 \: g} = 0.9 = 90 %\)
So, the percent yield of the reaction is 90%