CHEM 6A Lecture 3: CHEM W1 Lec 10/4/17
3 Jun 2018
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Addition and Subtraction
● The result cannot be more precise than the measured quantity with the least
precision (for all calculations)
● The number of decimal places in the result should be the same as the number of
decimal places in the measured quantity with the fewest digits after the decimal
● Fewer decimal places does not mean less precision
Ex. 3 samples of substances are weighed to give the values 9.68g, 37.019g, and 382.5g. Find
the total mass
● Add to get 429.199g
● 9.68 has the least number of significant figures, but 382.5 is the least precise
○ So our answer should only have one number after the decimal
● Sum = 429.2g
Ex2. A length of 970m was cut off from a string measured to be 3022m long, what is the length
of the remaining segment?
● 3022m - 970m = 2052m
○ 970 is less precise, so we round to the tens place
● 3.022*103m - 0.97 * 103m = 2.053*103m
● Answer = 2.05*103m = 2050m
Multiplication and Division
● The number of significant figures in the result should be the same as the smallest
number of significant figures in the data
Ex. Density = mass = 27.6g = .7888571428 g/mL = 0.79 g/mL
Volume 35mL
● 35 is less precise than 27.6, so answer should have 2 sig figs
● For multiple steps of calculation, use 0.788 g/mL
○ Used to carry over to prevent rounding error
Temperature Units
● Must know conversion between Kelvin and degrees Celcius - note that Kelvin
and Celsius scale are the same
● +1K = +1°C
● 0°C = 273.15 K
● K = °C + 273.15
● °C = K - 273.15
● °C vs °F (see textbook p.11 ex 1-3; use for HW only)
Basic SI units
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CHEM 6A Full Course Notes
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Document Summary
The result cannot be more precise than the measured quantity with the least precision (for all calculations) The number of decimal places in the result should be the same as the number of decimal places in the measured quantity with the fewest digits after the decimal. Fewer decimal places does not mean less precision. 3 samples of substances are weighed to give the values 9. 68g, 37. 019g, and 382. 5g. 9. 68 has the least number of significant figures, but 382. 5 is the least precise. So our answer should only have one number after the decimal. 970 is less precise, so we round to the tens place. 3. 022*103m - 0. 97 * 103m = 2. 053*103m. The number of significant figures in the result should be the same as the smallest number of significant figures in the data. Density = mass = 27. 6g = . 7888571428 g/ml = 0. 79 g/ml. 35 is less precise than 27. 6, so answer should have 2 sig figs.
