Chemistry NOTES Measurement

Objectives I can… Explain why standards are important Tell where standards come from Determine the level of uncertainty for measurements and calculations Explain the difference between accuracy and precision. Identify common sources of error

Need for Measurement Standards “Weights and measures may be ranked among the necessaries of life to every individual of human society. They enter into the economical arrangements and daily concerns of every family. They are necessary to every occupation of human industry; to the distribution and security of every species of property; to every transaction of trade and commerce; to the labors of the husbandman; to the ingenuity of the artificer; to the studies of the philosopher; to the researches of the antiquarian; to the navigation of the mariner, and the marches of the soldier; to all the exchanges of peace, and all the operations of war. The knowledge of them, as in established use, is among the first elements of education, and is often learned by those who learn nothing else, not even to read and write. This knowledge is riveted in the memory by the habitual application of it to the employments of men throughout life.” JOHN QUINCY ADAMS - Report to the Congress, 1821

Why are standards of measurement so important to keep?

Standards are Important By definition, a kilogram is the mass of a liter of water at the ice point (i.e. essentially 1 kg). 

Standards are Important The standard kilogram is housed at the International Bureau of Weights and Standards near Paris. 

Standards are Important The NIST F-1 atomic clock is accurate to within one second every thirty million years.

Standards are Important The Standard for the Meter The astronomer Christian Huygens suggested that the meter be defined by the length of a pendulum having a period of one second Others favored a meter defined as one ten-millionth the length of the earth's meridian along a quadrant (one fourth the circumference of the earth).

Standards are Important The standard meter, on the weights and Measures building in Paris

Measurement Every measurement is subject to uncertainty!   Question: How do we account for uncertainty? Answer: we have to calculate the accuracy and precision of each measurement.

Taking Measurements Measurements are limited by the measuring device or instrument. How long is the branch below?

Measurement Precision of a measurement is determined by one half the smallest division of the measuring device. Ex: The smallest division on a ruler is 0.1 centimeter (1 mm), therefore the reading may be off by + or - 0.05 cm. Read as 23.7 cm +/- 0.05 cm

Measurement Precision: The degree of exactness Can also be thought of as how close several measurements are to each other. Running many trails will reduce the amount of human error. Ex: Measurements were taken to see how long it took for a penny to fall 1 meter.  Which set of measurements is most precise? Trial Time (s) T1 0.41 T2 0.49 Tave 0.45 Trial Time (s) T1 0.44 T2 0.40 T3 0.47 T4 0.45 Tave

Measurement Accuracy: How close a measurement is to an established value or standard. Ex: 1 ml of water @ 3.98oC = 1.000 g If three measurements were taken: 0.989 g, 1.020 g and 0.991 g, which would be the most accurate?  0.991 g

Accuracy vs. Precision

How to Avoid Sources of Error Parallax – the apparent difference in position due to the angle of sight of the observer Observe measurements directly

How to Avoid Sources of Error In qualitative research bias is a major source of error. Bias is anything the researcher or the design of the research does to impact the answers given by participants.

How to Avoid Sources of Error Bias Questions Example of a leading question: “Some people think cola drinks are bad for you. What do you think?” Instead, frame the question neutrally. “What is your opinion about cola drinks?”

How to Avoid Sources of Error Biased Sampling A sample is a subgroup or segment of respondents you interview. A biased sample consists of respondents who don’t represent the group of interest. You interview the wrong people. Poor screening and recruiting causes biased samples. Screen in respondents you want; screen out those who don’t fit. Random sampling during recruiting reduces sample bias.

How to Avoid Sources of Error Example of bias sampling Nonresponse bias. Sometimes, individuals chosen for the sample are unwilling or unable to participate in the survey. Nonresponse bias is the bias that results when respondents differ in meaningful ways from nonrespondents. The Literary Digest survey illustrates this problem. Respondents tended to be Landon supporters; and nonrespondents, Roosevelt supporters. Since only 25% of the sampled voters actually completed the mail-in survey, survey results overestimated voter support for Alfred Landon. The Literary Digest experience illustrates a common problem with mail surveys. Response rate is often low, making mail surveys vulnerable to nonresponse bias.

Assessment Can I… Explain why measurement standards are so important for a civilization. determine the level of uncertainty for measurements and calculations Explain the difference between accuracy and precision. Identify common sources of error