## I got my midterm back and have a several question. Help me!!

1. Using the rules of significant figures, calculate the following 6.167 + 70
a) 76 b) 80 c) 76.167 d) 77 e) 76.17

In addition and subtraction, when you determine the significant figues, you look for the least decimal place, right? After calculate this problem, you would get 76.167 so if you were to change this to the least decimal place, wouldn't it be 76? I put A as an answer with a confidence and I got it WRONG!! The correct answer was b. Please explain. Thank you!!

2. Which of the following involves a chemical change?
a) boiling water b) melting ice c) chopping wood
d) cooking an egg e) none of these
I put E as an answer, but the correct answer is D.
I thought "cooking an egg" is a physical change, not a chemical change. Can anyone explain? Thank alot.

3. Which of these is a chemical property?
a) Ice melts at 0 degree Celcius.
b) Oxygen is a gas.
c) Helium is very nonreactive.
d) Sodium is a soft, shiny metal.
e) Water has a high specific heat.

4. In a chemical change,
a) a phase change must occur
b) the original material can never be regenerated
c) a phase change never occurs
d) the products are different substances from the starting materials.

Please explain. I put A as an answer. Can anyone tell me why not A but D?? Thanks in advance.

5. A solution can be distinguished from a compound by its
a) variable composition b) liquid state
c) heterogeneous nature d) lack of color

I put C as an answer. Can anyone explain???

6. The law of constant composition applies to
a) metals b) metalloids c) homogeneous mixtures
d) heterogeneous mixtures e) compounds

7. In chemistry, a formula is used to represent
a) a compound b) an element c) a metal
d) all of these e) none of these

8. The average mass of a boron atom is 10.81. Assuming you were able to isolate only one boron atom, the chance that you would randomly get one with a mass of 10.81 is
a) 0% b) 0.81% c) about 11% d) 10.81% e) greater than 50%

I put C as an answer. Can anyone explain why A?? Thanks.

9. Choose the pair in which the components have the same charge.
a) a proton and a hydrogen atom
b) a hydrogen atom and an electron
c) a neutron and a hydrogen atom
d) an electron and a neutron

I put A as an answer. I thought it as having + charge for both proton and hydrogen, no? Please explain. Thanks.

THANK YOU SO MUCH. YOU DON'T HAVE TO ANSWER ALL OF THEM ALL AT ONCE. YOU MAY ANSWER THE ONES YOU KNOW.
HAVE A GOOD ONE!

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### so, does a hydrogen atom

so, does a hydrogen atom have a nuetron at all?

### Re: I got my midterm back and have a several question. Help

[quote="binoo"]1. Using the rules of significant figures, calculate the following: 6.167 + 70
a) 76 b) 80 c) 76.167 d) 77 e) 76.17

In addition and subtraction, when you determine the significant figues, you look for the least decimal place, right? After calculate this problem, you would get 76.167 so if you were to change this to the least decimal place, wouldn't it be 76? I put A as an answer with a confidence and I got it WRONG!! The correct answer was b. Please explain. Thank you!![/quote]
A sneaky question this one! HAd me stumped for a second. BUT, if you stick 100% to sig fig rules, 70 could be any number from 65-74, to 1 significant figure. Therefore your final answer must be rounded to ONE significant figure - 76 is TWO significant figures. So your thinking was flawless, you just fell for the sneaky trick. In real life it would be made clear that 70 was rounded to 1sf, and not 2sf as is automatically assumed.

[quote="binoo"]2. Which of the following involves a chemical change?
a) boiling water b) melting ice c) chopping wood
d) cooking an egg e) none of these
I put E as an answer, but the correct answer is D.
I thought "cooking an egg" is a physical change, not a chemical change. Can anyone explain? Thank alot.[/quote]
Surely a physical change *is* a chemical change?
Cooking an egg is not a change of state - you heat it up and it goes from liquid to solid. So something is making the liquid solidify, a chemical reaction!

[quote="binoo"]3. Which of these is a chemical property?
a) Ice melts at 0 degree Celcius.
b) Oxygen is a gas.
c) Helium is very nonreactive.
d) Sodium is a soft, shiny metal.
e) Water has a high specific heat.

a, b, d and e are all just stating physical facts about the species. C is a chemical thing, because it is talking about how reactive it is - describing its *chemical* reactivity. Although that is an ambiguous question, which I would have answered incorrectly, because I'd think how chemical properties of all of those things can *explain* those physical facts.

[quote="binoo"]4. In a chemical change,
a) a phase change must occur
b) the original material can never be regenerated
c) a phase change never occurs
d) the products are different substances from the starting materials.

Please explain. I put A as an answer. Can anyone tell me why not A but D?? Thanks in advance.[/quote]
The answer is not A quite simply because a phase change does not have to occur - there are millions of reaction that happen without phase changes. OK, most reactions involving [i]solid[/i] things will involve another gas or liquid, so arguable will have a phase change of something somewhere, but there are millions of gas phase reactions - reactants are gas, products are gas. Phases are just physical properties, dictated by the enviromental conditions and the properties of the chemicals. They depend on what the chemical *are*, not wether or not a reaction has happened.

Whereas D is a nice, simple answer to this. A reaction occurs. If the products were the same as the starting material, then nothing will have happpened to the molecules, thus no reaction. A reaction is a change of the chemical make up. [b]Not[/b] a phase change, which is just a physical change!

[quote="binoo"]5. A solution can be distinguished from a compound by its
a) variable composition b) liquid state
c) heterogeneous nature d) lack of color

I put C as an answer. Can anyone explain???[/quote]
Well, a solution is defined as: "a homogeneous mixture of one or more substances (the solutes) dissolved in another substance (the solvent)."

[quote="binoo"]6. The law of constant composition applies to
a) metals b) metalloids c) homogeneous mixtures
d) heterogeneous mixtures e) compounds

Definition: "A relation stating that the relative masses of elements are fixed in a given chemical substance."
A homogenious mixture can have different compositions.
A specific compound can only ever follow that rule.

I've never heard of that law myself (is it an american school thing?) It's kinda pointless - what happens with the random percentages of different isotopes!!

[quote="binoo"]7. In chemistry, a formula is used to represent
a) a compound b) an element c) a metal
d) all of these e) none of these

A metal and an element cannot be described by a formula - they are just one thing, that is all. A formula is telling you how much of each element is in a specific compund.

Again, though, an ambiguous question - O[size=8]2[/size] is a formula (I would have said) but it's not a compound - defined by the fusion of TWO or more DIFFERENT elements.

[quote="binoo"]8. The average mass of a boron atom is 10.81. Assuming you were able to isolate only one boron atom, the chance that you would randomly get one with a mass of 10.81 is
a) 0% b) 0.81% c) about 11% d) 10.81% e) greater than 50%

I put C as an answer. Can anyone explain why A?? Thanks.[/quote]
10.811 is an [i]average[/i] of all of the possible atomic masses of different isotopes of Boron. Each atom has to be an integer atomic weight (proton weighs 1, Neutron weighs 1)

[quote="binoo"]9. Choose the pair in which the components have the same charge.
a) a proton and a hydrogen atom
b) a hydrogen atom and an electron
c) a neutron and a hydrogen atom
d) an electron and a neutron

I put A as an answer. I thought it as having + charge for both proton and hydrogen, no? Please explain. Thanks.[/quote]
A Proton has charge +
A Hydrogen ATOM - 1 proton (+) and one ELECTRON (-) - has an overall charge of zero. If it was a Hydrogen ION then it would have a charge of +1 (and it would also be exactly the same as a proton :))
A neutron has no charge. A Hydrogen ATOM has no overall charge, because the proton and electrons chaarges cancel each other out.

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