The Basics

Percentages are an important part of chemistry, it is probably one of the most common ways we have to express comparisons between quantities.  Although percents are not discussed continually in Chemistry 1 Honors, you will see it from time to time.  If interested in pharmaceutical science, chemical percents are consistently used.  So with that being said, it is a skill that needs to be understood before class begins.

NEED TO KNOW:
Percent actually means hundredths, or parts per hundred.  To convert a percentage to a decimal number, divide the percent by 100.  This is the same as moving the decimal point two places to the left.  Converting decimal numbers to percentages is the reverse of converting percents to decimals.  You multiply by 100.  Fractions can also be expressed as percentages.  To convert a fraction to a percent, first divide your numerator with your denominator, then multiple by 100.

Example:  56% is .56 as a decimal or 56/100 as a fraction.

Example: 3/4 cm is .75 cm as a decimal or 75%

Example: 6/15 L is  0.4 L as a decimal or 40%

Example related to your chemistry class: One mole of sucrose (C₁₂H₂₂O₁₁

- sugar) has a mass of 342.34 grams.  If 144.12 grams is carbon, 22.22 grams is hydrogen, what percent of this sample is oxygen? 

Answer:  First you would have to add up the two masses given to you of carbon and hydrogen, and then subtract this from the total mass of one mole.  This leaves you with the mass of your unknown = 176 grams is oxygen.  Then 176/ 342.34 (part over a total = fraction of your mass) = .5141 part oxygen.  Finally multiply by 100 to get your answer into a percent.  51.4%

Another example related to chemistry:  A 367.1 g sample of an unknown compound is found to contain 27.74% Mg, 23.56% P and 48.70% O.  What is the mass of each element in the compound?

Answer:  101.8 g Mg, 86.49 g P, and 178.8 g O

Use of scientific calculators and order of operations are also a skill that is necessary for success in chemistry.  Equations may contain only two numbers joined by one operation, but in chemistry often you have more complex calculations which contain multiple operations.  Sometimes you may see parentheses within your problem indicating that 'it' should be performed first.  To simplify an expression with more than one operation:  1)  Simplify any expression within grouping symbols ().  If one set of operations is inside another set of (), simplify the innermost expression first. 2)  Do all multiplication and division in order from left to right.  3) Do all addition and subtraction from left to right.  Most scientific calculators operate with algebraic logic.  However, in chemistry sometimes to get the correct answer you my need to tell your calculator what to do separately before getting an overall answer by putting in (). 

Example related to chemistry:  (Thermochemistry)

[(-241.8 kJ) + (-1206.9 kJ)] - [(-986.1 kJ) + (-393.5 kJ)]

Answer: -69.1 kJ

Scientific notation is another skill needed to be successful in chemistry.  In chemistry you work with numbers that are very large and small.  Avogadro's sonstant is 6.02213674000000000000000.  To avoid calculations errors, and to make working with the number easier we use sciencfic notation.  6.02 x 10²³ is how we would work with it.  The mass of an electron is 0.0000000000000000000000000000009109 kg, so obviously that just proves the point that you need to be able to work in scientific notation and that is also why you need a scientific calculator. 

What is the mass of an electron in scientific notation? 

9.109 x 10-31 kg.

Notice the large numbers have positive powers (exponents) and the small numbers have negative powers (exponents).

Being able to create and read graphical data is also required in chemistry.  Graphs are a useful tool for displaying scientific data because they show relationships.  There are four basic steps to constructing a graph (1) determine the independent variable (varied data) (2) determine the dependent variable (constant variable ex: time) (3) plotting the data, and (4) titling your work.  When preparing a graph, the scale of the axes should be chosen to include all data points and allow as much room as possible on both axes.  Each axis should be evenly divided with plenty of space between divisions, making the graph easy to read.

Make sure you can use the metric system for chemistry honors.  King Henry Died Monday Drinking Chocolate Milk.  k = kilo, h = hecto, D (dc or dk) = deca. M = meters (any base unit like grams, liter  joule too) d = deci, c = centi, and m = milli.  Be able to convert within the metric system, shemistry often requires you to do this.  It is a matter of moving the decimal place.  For example, 301859 joules of energy is 301.9 kilojoules (kJ).  Chemistry again will require you to know other metric prefixes like nanometer, this is what wavelength is measured in.

Be able to rearrange an algebraic equation to solve for an unknown variable.  For example the equation for density is D = M/V

D = density           M = mass          V = Volume

If I asked you to solve for M, you need to know that you would just multiply D and V to calculate M.

If I asked you to solve for V, you need to know that you would divide M by D.

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