Friday, January 28, 2011

Anouncements : Quiz Monday

Today in class : Today in class we learned stoichiometry. The basic idea is to use a balanced equation to figure out how much of one substance you would need to react with another. The simplest way to explain it is with the sandwich analogie. Lets say the basic recipe for a ham sandwich is 2 pieces of bread (B), 1 piece of cheese (C), and 3 pieces of ham (H). This equation would look like this: 2B+C+3H -> BCH. Using this equation we could find out how many sandwiches we could make with 6 pieces of ham or how many pieces of cheese we need if we have 14 pieces of bread. With the same equation we could also figure out how many pieces of bread we would need for 3 sandwiches. With smaller and more basic numbers like these we can solve the equation in our heads without much effort; however, when we have larger amounts like 33 pieces of ham these equations become more difficult. If we were trying to find out how many pieces of bread we need with those 33 pieces of ham we would have to multiply it by the ration of bread to ham given to us in the original equation. In this case it is 2 pieces of bread over 3 pieces of ham. In order to solve this we need to take the starting information of 33 pieces of ham and multiply it by the ratio 2 pieces of bread over 3 pieces of ham. It is important to remember that when you set up the problem that the same units always cancel out. That is why the 3 pieces of ham must be on the bottom in order to cancel out the 33 pieces of ham given to us in the starting piece of information. Now in chemistry, instead of sandwich ingredient we use molecules and elements. The ideas however are exactly the same. Take a balanced equation and use the information given to find the amount needed to cause a chemical reaction. We also learned how to find the exact mass of elements that we would need in order to cause a reaction. Back to the sandwich analogise, if a piece of bread weighs 2 grams, a piece of cheese weighs 1 gram, and a piece of ham also weighs 1 gram then we can find out the weight of a sandwich ingredient needed to make the sandwich. If we had 2 grams of cheese, in order to find the amount of ham needed we would multiply the starting amount by the amount of cheese needed to make 1 sandwich which is 1. So 2 grams of cheese equals 2 pieces of cheese. To convert that to ham we would multiply that by the ratio of ham to cheese 3pieces of ham over 1 piece of cheese. this gives us 6pieces of ham because the cheese cancels out. Finally to find the amount of ham in grams we simply multiply the 4 pieces of ham by the 1 gram each piece of ham weighs. When you put all this together you it ends up meaning that for 2 grams of cheese you need 6 grams of ham to make the sandwiches. Again, in chemistry we use the same concept but instead of grams of ham we use the molar masses to find the amount of a substance needed to cause a chemical reaction.

Homework : Pages 15 and 17