Homework 2 due this Wednesday- in class Homework 3 posted due Wednesday 7 Feb
Fong lab: Answer Pre-lab questions* page 13 of Lab Manual on separate piece of paper and bring to lab tomorrow morning • * • What is the density of pure water at typical lab temperatures? • Explain finding “mass by difference.” • What is a pycnometer ? • How can the exact volume of the pycnometer be determined ? Lab Notebook must have Purpose and Procedure written out before hand. (Will check at door.)
Corrections/additions to Procedure on page 14 4. Remove the marble , measure the mass of the full pycnometer and record in the table. 9. Remove the marble, measure the mass of the full pycnometer and record in the table. • 14. Measure the mass of the empty pycnometer and record in the table • 15a. Add about half the volume of the unknown solid to the pycnometer • (Erlenmeyer) • 15b. Measure the mass of the unknown solid and pycnometer. • Fill the Erlenmeyer completely with water, then place the marble on • top. A small amount of water should spill out. • Remove the marble, measure the mass of the full pycnometer with theand record in table.
How many 0.6 Mm=___km ? • 600 km • 6*10-3 km • 6*106 km • 6000 km
5 ps = ____ns ? • 5*10+3 ns • 5*10-6 ns • 5*10+2 ns • 5*10-3 ns
PRECISION AND ACCURACY : what are they ? Your thoughts…..
Why do we care about Sig Figs in Chemistry? The Sig Fig count tells us the precision of measurement How long is the screw ? 5 cm 1 sig fig How long is the screw now? 5.1 cm 2 sig fig Nerdy detail: actually you can add one more sig fig to each since we can- in principle- guess another decimal place between hash marks…(the ruler is `analog’)
Metric, digital micrometer does even better 51.03mm How long is the screw now in cm ? 5.103 cm How many sig figs now ? 4 sig fig Nerdy detail: since the above instrument is digital, not analog- what you see is what you get. No more extra sig figs here.
Technological aside: Higher verifiable precision (more sig figs) correlates with higher technical development 1492: Columbus is within + 15,000 miles of estimate for distance to China from Spain 2016: we can pin point the position of anyone via GPS to within ~ + 0.0001 miles (within 12 inches) In ~ 500 years human measurement precision has increased by > 10 million
Counting sig figs: Atlantic& Pacific method Absent Decimal… move from right (Atlantic side) and start counting when first non-zero digit encountered 10020 300 450070 4 1 5 # sig fig ?
Counting sig figs: Atlantic& Pacific method Possesses decimal …move from left (Pacific side) and start counting when first non-zero digit encountered 4 1.002 0.030 0.00400 2 # sig fig ? 3
What is the sig fig count given a scientific notation version of a number ??? 3 sig figs exponent Example: 6.02*1023 Pre-exponent The count of digits in the pre-exponent is the sig fig count.
# sig figs ? 2 2 3 5 2