Me again, I am helping to implement a chemistry class and we are still struggling with some sig fig issues. I am not a chemist or even a chemistry teacher and have just been working hard to understand it all. So please excuse the naive questions.
In this question are there 2 sig figs despite have to change the 50°C to K. You can create more accuracy by converting to Kelving correct?
If 77.0 mL of nitrogen gas (N2) is collected over water at 50°C and 763 mm Hg, what is the mass of the dry nitrogen gas collected? The vapor pressure of water at 50°C is 92.5mmHg.
50°C
Here is the student's answer:
763 mm Hg = 92.5mmHg + pressure of N
pressure of N2=763 mm Hg-92.5mmHg
Pressure of N2=670.5 mmHg
Convert stuff
Convert temperature and volume
77.0 ml *1 L/1,000mL
50.0°C + 273 = 323K
P = 670.5 mmHg
V= 0.077 L
n = ?
R = 62.4 L mmHg/mol K
T = 323K
Convert PV=nRT
n=PV/RT
Insert numbers
n= 670.5 mmHg*0.077L
—-------------------------
323K*62.4 L mmHg/mol K
670.5*0.077
—-------------------------
323*62.4/mol
0.00256155 mol N2
N2 =14.007 * 2 = 28.014g/mol
0.00256155 mol N2 *28.014g/mol
0.071759 g N2
Three sig figs
Answer in both grams and mols because I don’t know what she wants
0.00256 mol N2 or 0.0718g N2
