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# Tutor profile: Amanda L.

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Amanda L.
Teacher for 20 years
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## Questions

### Subject:Basic Chemistry

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Question:

What is the limiting reactant in a reaction and how do I determine that?

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Amanda L.

When a chemical reaction happens and two or more substances combine, one substance will get used up first. It's like saying, if I am building hamburgers and I have six burgers and eight buns, I can only build six burgers because I am going to run out of burgers first. The buns are said to be in excess, meaning I have some left over. In a chemical reaction, one reactant gets used up and the reaction will stop and the other reactant will have some left over, or excess. In order to determine the amount of limiting, you must not only look at how much you have, but also the ratio to which it gets used in the reaction. There are three simple steps to follow to find limiting reactant. Step 1: Convert all reactants from grams to moles (if it's already in moles then skip this step) Step 2: Divide all by the least number of moles Step 3: Divide by the coefficient from the balanced equation (the smallest will be your limiting) Here's an example If 6.2 grams of Mg(OH)2 reacts with 4.5 grams of HCl according to the following equation, which reactant will be the limiting reactant? Mg(OH)2 + 2HCl --> MgCl2 + H2O Step 1: convert all reactants to moles using the periodic table 6.2 g Mg(OH)2 X 1 mole/58.3188 g = 0.1063 4.5 g HCl X 1 mole/36.4609 g = 0.1234 Step 2: Divide both by the smallest (which is 0.1063) Mg(OH)2 = 0.1063/0.1063 = 1 HCl = 0.1234/0.1063 = 1.16 Step 3: Divide by the coefficient (number of moles) from the equation Mg(OH)2 = 1/1 = 1 HCl = 1.16/2 - 0.58 <-- this is the smallest therefore HCl is the limiting reactant This means you will run out of HCl in the reaction and Mg(OH)2 will have some left over, or be in excess.

### Subject:Physics

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Question:

If you a tree falls in the woods and no one is around to hear it, does it make a sound?

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Amanda L.

This is an interesting question and one that most people do not realize because it depends on the actual definition of sound. Sound is how a persons ears perceive vibrations. In order for sound to occur a person must interpret vibrations. So when the tree falls, it does create vibrations in the air. This energy is passed through the air from one molecule to another as they collide but it isn't until those vibrations reach the ear and tiny hairs inside the ear vibrate. When those hairs vibrate, the brain interprets those vibrations into sound. So the correct answer is no, there is no "sound," just vibrating particles in the air.

### Subject:Physical Science

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Question:

Why can a bird sit on a power line of high voltage but not be electrocuted?

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Amanda L.

Electricity is the flow of electrons from one place to another. But in order for the electrons to flow you have to have a difference in potential energy. Potential difference is like saying one end is high and one end is low. Much the same way that water flows down hill, from a high potential energy to a low potential energy. When the bird sits on the wire, both feet are at the same potential therefore no electricity flows through the bird. However, if the bird were to hike one leg up on a pole where there was no voltage and the other leg on the wire, at a high voltage, the electricity would flow through the bird and therefore the bird would be toasted.

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