CALCULATIONS IN CHEMISTRY

  1. Accuracy and Precision
  1. Accuracy:
  1. Precision:
  1. Accuracy
  1. Again, accuracy is how close a particular measurement is to the correct value

-Example:

  1. How do we know if a measurement is accurate enough? (HONORS)

-Accuracy is indicated by calculation of percent error:

  1. Learning Check (HONORS)

-Let’s go back to the example. As mentioned before, the actual height of the door is 210 cm.
Measurement A is 209 cm. Calculate the percent error.

-Again, let’s go back to the door example. With your partner, try to find the percent error of measurement B.

-For the percent error, the smaller the percent error, the better.

  1. Precision
  1. Reproducibility
  1. Accuracy vs Precision
  1. Learning Check

-For each diagram, label it as high/low accuracy AND high/low precision

-With a partner, create a Venn diagram of accuracy vs precision below. If you are a visual learner, you may also include images in your Venn diagram.

  1. Why is it important to know the difference between accuracy and precision?
  1. Significant Figures
  1. Scientists use…

  1. Learning Check

-How many significant figures are there in the following measurements:

  1. 304 g
  2. 0.000405 g
  3. 44.0 g
  4. 6.034 mL
  5. 0.000009 cm
  6. 3.0001 km
  7. 96.00 grams
  1. Calculations with Significant Figures

-Remember: your calculator is DUMB!!

-It does not understand sig figs!

-Do not just write down all the digits your calculator gives you.

-Think for yourself!

  1. SIG FIGS: MULTIPLICATION AND DIVISION
  1. Learning Check

-What is 6.9 m x 11.4 m (written to the correct number of significant figures)?

-What is 0.0032 x 204.0 (written to the correct number of significant figures)?

-What is 0.204367 divided by 36 (written to the correct number of significant figures)?

  1. SIG FIGS: ADDITION AND SUBTRACTION
  1. Learning Check

-What is 3.456g + 4.56g + 5.6g? Use the correct number of significant figures.

  1. Scientific Notation

•Chemists often make measurements using very large or very small numbers

Examples:

•The mass of one gold atom is .000 000 000 000 000 000 000 327 grams.

•One gram of hydrogen contains 602 000 000 000 000 000 000 000 hydrogen atoms.

•If you remember from our measurement notes, scientists do not like to count a lot of zeroes

•Hence,

  1. SCIENTIFIC NOTATION RULES
  1. The first part
  1. The second part
  1. REMINDER: POSITIVE EXPONENTS

-Reminder:

When converting from ordinary notation to scientific notation….

for a POSITIVE exponent…

move the decimal point to the LEFT.

63000 grams = 6.3 x 104 grams

-The opposite problem

When converting from scientific notation to ordinary notation….

For a POSITIVE exponent...

move the decimal point 4 places to the RIGHT

6.3 x 104 = 63 000 g

  1. Learning Check

-Convert 745000g to scientific notation

  1. REMINDER: NEGATIVE EXPONENTS

-Reminder:

When converting from ordinary notation to scientific notation….

for a NEGATIVE exponent…

move the decimal point to the RIGHT.

0.00045 grams = 4.5 x 10-4 grams

-The opposite problem

When converting from scientific notation to ordinary notation….

For a NEGATIVE exponent...

move the decimal point 4 places to the LEFT

4.5x 10-4 grams = 0.00045 grams

  1. Learning Check

-Convert the charge of electron to scientific notation

Charge of electron = 0.00000000000000000016 C

HELPFUL YOUTUBE LINKS:

ACCURACY AND PRECISION:

SIGNIFICANT FIGURES:

(Review)

MULTIPLYING/DIVIDING SIGNIFICANT FIGURES:

ADDING/SUBTRACTING SIGNIFICIANT NUMBERS:

SCIENTIFIC NOTATION: