We can consider the hydrogen gas of the first trial like an ideal gas and use the ideal gas law.
P * V = n * R * T
Where P is the pressure of the gas, V is the volume, n is the number of moles, R is the ideal gas constant and T is the temperature. We already have those values.
P = 753.8 mmHg (Pressure of Dry Hydrogen Gas)
760 mmHg = 1 atm
P = 753.8 mmHg * 1 atm/(760 mmHg)
P = 0.9918 atm
V = 40.3 mL (Volume of Hydrogen Gas Collected)
1000 mL = 1 L
V = 40.3 mL * 1 L/(1000 mL)
V = 0.0403 L
R = 0.082 atm*L/(mol*K) (ideal gas constant)
T = 295.6 K (Temperature in K)
Finally we can replace these values into the formula and solve it for n.
P * V = n * R * T
n = P * V /(R * T)
n = (0.9918 atm * 0.0403 L)/(0.082 atm*L/(mol*K) * 295.6 K)
n = 0.00165 moles
Answer: the number of moles of Hydrogen gas collected in trial 1 is 0.00165 mol.
If we use R = 62.358 L-torr/mol-K we need:
T = 295.6 K
V = 0.0403 L
P = 753.8 mmHg = 753.8 torr
n = P * V /(R * T)
n = (753.8 torr * 0.0403 L)/(62.358 torr*L/(mol*K) * 295.6 K)
n = 0.00165 moles
