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# Making Measurements and Using Numbers - PowerPoint PPT Presentation

Making Measurements and Using Numbers. The guide to lab calculations. Not just numbers.

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### Making Measurements and Using Numbers

The guide to lab calculations

• Scientists express values that are obtained in the lab. In the lab we use scales, thermometers and graduated cylinders to record mass, temperature and volume. It may seem simple to read the instruments but it is actually more difficult than you think.

• Thermometers measure temperature. Key points:

• Temperature is read from the bottom to top.

• Lines on a thermometer are only so accurate. We as scientist are allowed to estimate between the lines.

• The unit on thermometers is Celsius

• We are allowed to estimate one additional digit to make the reading more significant.

• No matter what the last line of reading may be on the thermometer, you may estimate one additional digit (with a few exceptions)

• When markings go up or down by ones, estimate your measurement to the tenths place

• When markings go up or down by tenths, estimate your measurement to the hundreths place

• When markings go up or down by 2 ones or 2 tenths, estimate your measurement to that place

• Some errors or uncertainty always exists in measurements. The measuring instruments place limitations on precision.

• When using a device we can be almost certain of a particular number or digit. Simply leaving the estimated digit out would be misleading because we do have some indication of the value’s likely range.

• Because of certain physical properties, liquids are attracted or repelled from glass surfaces. Water is especially attracted to glass. Due to this attraction a meniscus forms when water is in glass tubing.

• Meniscus is the upside down bubble that forms when water is in glass

• When reading glass volumes, the volume is of liquid is read at the bottom of the meniscus.

• Not only is the liquid read at the bottom of the meniscus but the last digit of the reading is estimated.

• The estimation tips are the same for all measuring devices

• No matter the guess you are right. As long as you include the guess in your answer

Water Meniscus at the bottom of the meniscus.

Significant figures at the bottom of the meniscus.

• In science, measured values are reported in terms of significant figures.

• Significant Figures in a measurement consist of all the digits known with certainty plus one final digit, which is somewhat uncertain or is estimated.

Why Use Sig Figs? at the bottom of the meniscus.

• We can only measure as well as our equipment

• We cannot make estimates without being precise

• Estimating multiple measurements can add up to a lot of error

Rule 1 don’t write down all the numbers the calculator displays.

• All non-zero numbers are significant.

• Examples:

• 123 L has 3 significant figures (sigfigs)

• 7.896 m3 has 4 sig figs

• 8 meters has 1 sig figs

Rule 2 don’t write down all the numbers the calculator displays.

• Zeros appearing between non zero digits are significant.

• Examples:

• 40.7 L has 3 sig figs

• 87,009 km has 5 sig figs

Rule 3 don’t write down all the numbers the calculator displays.

• Zeros appearing in front of all non zero digits are not significant.

• Examples

• 0.095897 m has 5 sig figs

• 0.0009 kg has 1 sig fig

• These zeros are place holders

Rule 4 don’t write down all the numbers the calculator displays.

• Zeros at the end of a number AND to the right of a decimal point are significant.

• Examples

• 85.00 g has 4 sig figs

• 9.000000000 mm has 10 sig figs

Rule 5 don’t write down all the numbers the calculator displays.

• Zeros at the end of a number but to the left of a decimal point may or may not be significant. If a zero has not been measured or estimated but is just a placeholder, it is not significant. A decimal point placed after zeros indicates they are significant

• Examples

• 2000 m has only 1 sig fig

• 2000. M has 4 sig fig (decimal at the end)

When To Apply Sig Fig Rules? don’t write down all the numbers the calculator displays.

• Sig fig rules only apply to situations where a measurement was made by an instrument.

• For all other situations, all measurements are exact, and therefore contain an unlimited amount of significant figures.

300 mL = 1 sig fig

300 people = 3 sig figs

300 pennies = 3 sig figs

Calculations with Sig Figs don’t write down all the numbers the calculator displays.

• When multiplying and dividing, limit and round to the the number with the fewest sig figs.

• 5.4 x 17.2 x 0.0005467 =?

• How many sig figs are in the number 23000000000?

• Do we need to write all of the zeros?

• Scientist often deal with very small and very large numbers, which can lead to a lot of confusion about counting zeros.

• Scientist notation takes the from of

M x 10n where 1 <M<10 and “n” represents the number of decimal places moved.

• 150000 becomes 1.5 x 10 the 5

• 43500000 becomes 4.35 x 107

• 0.0034 becomes 3.4 x 10-3

• 0.000000000005687 becomes 5.687 x 10-12

More examples…

• Ex: (4.58 x 105) (6.8 x 10-3)

• Multiply the bases

• Adjust value to correct scientific notation format

• Determine sig figs from quantities listed in the original problem

• Ex: (2.8 x 10 the -5) / (1.673 x 10-2)

• Divide the bases

• Subtract the exponents

• Adjust value to correct scientific notation format

• Determine sig figs from quantities listed in the original problem

• Ex: (3.52 x 106) + (5.9 x 105) – (6.447 x 104)

• Convert all quantities so that they all have the same largest exponent

• Add or subtract the base numbers

• Adjust value to correct scientific notation format

• Determine sig figs from quantities listed when all exponents have been adjusted.

• Accuracy is the ability of a tool or technique to measure close to the accepted value of the quantity being measured (how close it is to being right)

• Precision is the ability of a tool or technique to measure in a consistent way (how close the measurements are to each other)

• A student measured a magnesium strip 3 times and recorded the following measurements: 5.49cm, 5.48cm, 5.50cm

The actual length of the strip is 5.98cm. Describe the results in terms of accuracy and precision.

Density the

• Density is a mass to volume ratio

• D = m/v m = Dv v = m/D

• Density is an intensive property and will not change regardless of the amount of matter present.

• Each substance has its own defined density value ex. H2O = 1g/cm3

• You must know the mass and volume if you want to experimentally determine the density of a sample of matter

• Mass can be found using a scale (g)

• Volume can be found by one of two ways:

• For regular shaped objects, use a ruler to find l x w x h (measurement will be in cm3)

• For irregular shaped objects, use water displacement (measurement will be in mL)

• Remember… 1cm the 3 = 1mL

• Water displacement is a process in which an object is submerged in water. The difference between the water level before and after the object is submerged in the water will be the volume of the object

Percent Error… the How Wrong Are You?

• Once your densities are determined experimentally, you can then compare your lab results to the theoretical value by using the following equation:

• % error = (theoretical – experimental)

theoretical

X 100

Ideally, you would shoot for <5% error in any lab experiment

Theoretical values are given by teacher or text

• 78.6 mm + 68.350 cm =

• 55 L + 25 cm3 =

• The density of cork is .193 g/cm3. What is the mass in pounds of 7.0 x 103 mL of cork? (1 lb = 16 oz) , (1 g = .0353 oz)