$$PV = nRT$$
where:
* P is the pressure in atmospheres
* V is the volume in liters
* n is the number of moles
* R is the ideal gas constant (0.08206 L*atm/mol*K)
* T is the temperature in Kelvin
We need to convert the temperature to Kelvin:
$$T = 100.0 \degree C + 273.15 = 373.15 K$$
We also need to convert the pressure to atmospheres:
$$P = 940 \ torr \times \frac{1 \ atm}{760 \ torr} = 1.237 \ atm$$
Now we can plug in the values we know into the ideal gas law and solve for n:
$$n = \frac{PV}{RT} = \frac{(1.237 \ atm)(0.253 L)}{(0.08206 L*atm/mol*K)(373.15 K)}$$
$$n = 0.0100 \ mol$$
Therefore, 0.0100 moles of chloroform are required to fill the 253-mL flask at 100.0 degrees C and 940 torr.
