Your Percentages

In this chapter, you see how to figure percent increase and percent decrease and determine whether what you see advertised is a good deal. Everyone is affected by interest on money — whether you re borrowing or saving — sol include problems dealing with computing interest as well. 

The usual move from fractions to percents is through decimals — the decimal format is the middleman in the process. You probably already know some of the more common equivalences of percents and fractions. You know that 50 percent is equivalent to Vi and 25 percent is equivalent to A. Well, I m assuming that you know this, but, just in case, here are some properties and techniques that you can use to make the transitions easier. You don t have to memorize these properties, but having some of them in mind as you re working on percentage problems is helpful. 

Switching from fractions to decimals to percents and back again Investigating both the practical and impractical with percents Using percentages to your advantage — in your best interest 

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Remember that if you do not want to exclude missing values from your counts or from the denominators of your percentage calculations, you need to specify the MISSING option in your TABLES statement. 

Note If you rounded your percentages properly, they add to 100.1%, not 100%. If you do away with rounding, you get exactly 100%. Rounding is common practice, though, so don t be too worried if your answer is off by a tenth or two. 

SO3. Following the same procedure as in Question 7, you calculate 1.25 mol of sulfur and 3.75 mol of oxygen. You must change your percentages to mass and then divide by the molcir mass of each element. 

If your percentage yield was not 100%, suggest sources of error. 

Do some research to find out the percent by mass of iron in steel wool. Predict what your percentage yield would be if you had used pure iron in this reaction. Would it make a difference ... 

In this section, you have learned how the amount of products formed by experiment relates to the theoretical yield predicted by stoichiometry. You have learned about many factors that affect actual yield, including the nature of the reaction, experimental design and execution, and the purity of the reactants. Usually, when you are performing an experiment in a laboratory, you want to maximize your percentage yield. To do this, you need to be careful not to contaminate your reactants or lose any products. Either might affect your actual yield. 

In a reaction that produces a precipitate that you want to recover, what techniques would you use to maximize your percentage yield ... 

Ask your teacher for the theoretical mass of Na2C03 in the sample, and calculate your percentage error. 

How great was your error (9) What was your percentage error (10) Why was no correction made for the weight of air in the flask ... 

The yield of the product obtained by experiment, therefore, may be lower than expected sometimes it is a great deal lower. It is usual to report your percentage yield when you are doing preparative chemistry ... 

Analyzing Results Very seldom are chemists able to achieve a 100% yield of a product from a chemical reaction. However, the yield of a reaction is usually important because of the expense involved in producing less product. For example, when magne-siiun metal is heated in a crucible at high temperatures, the product magnesium oxide, MgO, is formed. Based on your analysis of the reaction, describe some of the actions that you would take to increase your percentage yield. The reaction is as follows ... 

For each experiment you did, what was your percentage yield, based on the limiting reagent Explain the possible reasons for any disagreement between your yield and the expected yield. 

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