Accuracy, Precision, & Percent's

Precision vs. Accuracy Precision- getting the same results again & again (reproducible) Accuracy- getting results close to the accepted or standard value Which do you think is better?

Precision vs Accuracy If the is the target or bulls-eye Which does this represent? High precision Low accuracy

Precision vs Accuracy How about this one? lower precision higher accuracy

Precision vs Accuracy And this one? high precision high accuracy

Accuracy vs. Precision So which is better? Accuracy Why? Well if you get the same results over and over but they are far off from the standard or accepted value then experiment is flawed. Percent Error shows accuracy. However if there is no known standard then your experiment also needs to be precise so that other scientists who preform your experiment should be able to get the same results repeatedly. A scientist’s goal is to be both accurate and precise.

Practice The following data was collected. If the volume of the gas was known to be 2.20 L, the data has- A) low precision and low accuracy B) low precision and high accuracy C) low accuracy and high precision D) high accuracy and high precision

Percent Yield & Percent Error 3 main types we use in chemistry: Percent Yield, Percent Error and Percent Composition Percent Yield tells you the percent of the product you obtained, the goal of course being 100% but we will have labs with as little as 10% and as much as 200% (trust me its possible but not good) Percent Yield is calculated by dividing what you got- called Experimental or Actual Yield by the Theoretical Yield- what you should have gotten x 100. % YIELD = Experimental Yield x 100 Theoretical Yield

Percent Error Percent Error tells you how accurate your results are. Percent Error = (Measured Value - Accepted Value) x 100 Accepted Value Now here is the confusing part. The measured value is the same thing as the experimental value & the accepted value can also be called the theoretical value. So to keep everything the same I tweak the 2 formulas to look like this

My fool-proof versions % YIELD = What’cha Got x 100 What’cha Should’a Got Percent Error = (What’cha Got - What’chaShould’a Got) x 100 This way they both similar and you are less likely to forget what goes where. I know it’s not very grammatically correct but trust me it works 

Practice: During a reaction between sodium bicarbonate (baking soda) and acetic acid (vinegar) 23.4 grams of sodium acetate was collected. The theoretical yeild should have been 33.6 grams. What is your percent yeild? During a density lab you got an experimental value of 15.5 g/cm3 , if the actual density of Au (gold) = 19.3 g / cm3