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Lauren L.
Certified Science Teacher (7-12)
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Basic Chemistry
TutorMe
Question:

How can a reactant that you have more of also be the limiting reagent? Shouldn't the reactant that you have less of be the limiting reagent since the supply is limited?

Lauren L.
Answer:

This is a common misconception in chemistry. Let's start by forgetting about moles for a second and think about sandwiches. Yes, sandwiches. It is the exact same concept but for some reason seems to resonate more because everybody has made a sandwich. If you had 8 pieces of bread and 6 pieces of ham, what would be your limiting reagent? Most chemistry students would say the ham because you have less of it. However, we know that it takes two pieces of bread for every one piece of ham to make a sandwich. Therefore, the bread in this problem would be your limiting reagent because you would only be able to make 4 sandwiches with 2 pieces of ham left over. Now change sandwiches back to moles. Simply looking at what reactant you have more of does not indicate which is the limiting reagent. You will need to calculate how much excess of each reactant you will have for every reaction in order to figure this out.

Biology
TutorMe
Question:

If sickle cell anemia is such a bad disease, why does it still show up in human populations? Shouldn't natural selection have selected against it so that it isn't passed on?

Lauren L.
Answer:

All living organisms want to do two things; survive and reproduce. Sickle cell is a chronic disease that causes misshapen red blood cells and leads to symptoms such as lack of oxygen and anemia. So why is it still present in human populations? It is important to note that sickle cell anemia is predominately seen in African and African American populations where malaria is common and life-threatening. While sickle shaped blood cells are not the most efficient for transporting oxygen around the body, they also provide some protection from malaria by providing a less hospitable environment for the malaria parasite. If populations with normal blood cells are more prone to malaria, then they would also be less likely to survive or pass on their genes for normal blood cells. Populations with the sickle cell trait, however, would have some protection from malaria and would be able to survive and pass on their genes for sickle shaped blood cells. While sickle cell anemia causes many heath problems, most are not life threatening and the individual is still able to survive and reproduce whereas individuals with malaria are far less likely to. Therefore, it would actually be evolutionarily advantageous to select for the sickle cell trait for protection against malaria in an evolutionary trade-off.

Physics
TutorMe
Question:

Alan leaves LA at 8am to drive to San Francisco 400 miles away. He travels at a steady 50mph. Best leaves LA at 9am and drives at a steady 60mph. Who gets to San Francisco first?

Lauren L.
Answer:

In order to solve this problem, you will simply solve for time twice; once for Alan and once for Beth. Both use the same constant acceleration equation, you just need to plug in their distance and individual speeds to find their times. Here is the constant acceleration equation: x -xo = vo t + ½ a t^2 Because the velocity is constant and therefore there is no acceleration, a=0. You can then simplify the equation to x - xo = vot. Now simply use the order of operations to solve for t. You will do this twice for both Alan and Beth to see which of them took less time.

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