AS.030.105 Study Guide - Midterm Guide: Burette, Jmol, Ideal Gas Law
Chem Lab Exam 2 Note Card info
Sig Figs:
• Percent Deviation: # of sig figs in % deviation is ALWAYS the # of sig figs in subtraction in
the numerator
o Formula:
!""
#
$%&'$()&*+,-+&.%$(
&*+,-+&.%$( /
• Percent Yield:
o Formula: 100(
$%&'$(01.+(2
&*+,-+&.%$(01.+(23
• Standard Deviation: leading digit of 1 = 2 sf, otherwise just 1 sf
o * look at “MAXIMUM” stuff on sig figs sheet (pg 1), these are limited further by
the decimal position of the least significant digit of std. dev.
o Formula:
0
4
0
5
678)7
9
3:
8
;<)=
(LINEST function in Excel used to determine std. dev. of
slope and intercept)
• Uncertainties in measurement:
o Reading digital measuring devices: the last digit shown is the least sig digit
o Reading graduated measuring devices: estimate the value to one decimal place
more than the gradation (0.1 mL increments on glass burette = recorded to .01
mL)
• Sig Fig Algebra Rules:
o All nonzero digits are significant (1.234 = 4 sf)
o Zeroes between nonzero digits are significant (1.02 = 3 sf)
o (0.012 = 2 sf)
o (0.120 = 3 sf)
o (100 = 1 sf)
o (1.0 x 102 = 2 sf)
o (100. or 1.00 x 102 = 3 sf)
o (100.0 or 1.000 x 102 = 4 sf)
o Addition/Subtraction: round off result to the leftmost decimal place (40.123 +
20.34 = 60.46), (4.25E5 + 3.23E3 = 4.25E5 + 0.323E5 = 4.28E5)
o Multiplication/Division: round off the result to the smallest # of sig figs (1.23 x
2.0 = 2.5, not 2.46)
o Exact # (60 seconds in a minute, 24 hours in a day, etc) never limit # of sig fig
(325 seconds/60 seconds = 5.42 minutes)
o Logarithms: log(12.8) = 1.107 (mantissa is .107 = 3 digits = 3 sf = 12.8), log(10.5) =
1.021 (mantissa is .021 = 3 digits = 3 sf = 10.5) *it is # of digits in mantissa, not sf!
o Exponents: 101.23 = 17 = 1.7E1 (2 digits = mantissa (.23)), 101.03 = 11 = 1.1E1 (2
digits = mantissa (.03))
o (2.44 x 2.0) + 2.96 = 4.88 + 2.96 = 7.84 = 7.8
o avg of 122 and 123 = 123
>
0.5
Exp 4:
Numbered Equations/concepts from manual:
• Ideal Gas Law: PV = nRT
o P = room pressure, T = temp. of boiling water (100˚C @ 1atm), V = weighing tube
filled with water and then using density to find V, R = gas constant (check units!)
• Van Der Waals Equation: ?
@ABC:
D:
E6
FGHI
3
JHKL
o Compare PV with ?
@ABC:
D:
E6
FGHI
3
o Molecules w/ strong intermolecular attractions will have larger values of a (polar
& large molecules w/ many electrons)
o Small molecules have smaller a values (hydrogen & inert gases)
o Bigger a = lower P:
@ J0 CMN
D)CO GBC:
D:
o b = V of 1 mol of liquid, bigger b = increased P
• Acetone used to rinse D. tube – dissolves remaining water, heat from hand forces
acetone out, dispose in red container
Tips:
• Must wipe fingerprints (weigh enough to cause significant error)
• Weigh empty Dumas tube @ beginning of lab (= tube + air)
• Clamp Dumas tube at top end & pack paper towels around top part of Dumas tube
(want to prevent liquid condensation in top of tube & immerse max amount of tube in
boiling water)
• Do not take Dumas tube out of boiling water to check liquid (bp of unknowns are only a
few degrees below bp of water and will recondense immediately)
• Use P, V, R, & T to find n (moles)
Prelab Answers:
• Weight of D. tube decreasing rapidly = water evaporating from outside of tube
• Weight of D. tube increasing rapidly = liquid condensing in tube as it cools, letting air
enter
• After heating to 100˚C and waiting, there is liquid in the tube = this is expected
Worksheet Answers:
• What is in D. tube at these times:
o TA added 0.5mL of unknown but before heated to 100˚C
§ A: Air, unknown vapor, unknown liquid
o After heated to 100˚C, just about to let it cool
§ A: unknown vapor
o After tube done cooling, about to weigh
§ A: Air, unknown vapor, unknown liquid
• TA adds 2mL of unknown liquid into D. tube instead of 0.5mL:
o A: after heated to 100˚C and cooled, mass of unknown = unchanged
• TA adds tiny drop of unknown liquid into D. tube for first trial:
o A: after heated to 100˚C and cooled, mass of unknown = too small
• Didn’t fill D. tube all the way w/ water?
o measured volume: too small
o # of moles calculated w/ V: too small
o molar mass calculated w/ V: too large
• Purpose of vacuum port trap?
o Prevents water from entering pump
