Chem help needed (again)

1. Hi everyone. I'm sorry to ask for help again but my professor isn't very helpful and we don't have class now until Monday when the assignment is due...

The question is: Gasoline has a density of about 0.67 g/mL. How much does 18 gallons weigh in kilograms?

I've done the conversion factors, and come up with an answer but I'm unsure of what to do with the stupid density figure??? Any help would be so nice. I'm just so confused with this stuff and I don't know what to do...

I can't really go in for help because I have 3 kids and my husband travels during the week. I am really having a hard time... not to mention a major headache.

Ugg... TIA
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Joined: May '06; Posts: 698; Likes: 229
SAHM

3. First, let's look at the fundamental properties of matter:
• mass - This is what we perceive as weight although weight is actually a derived unit of force (that is, mass times the acceleration of gravity = weight). Basic SI units of mass are grams (g).
• length - Pretty self-explanatory. The basic SI unit of length is the meter (m).
• Some others include temperature, charge, and spin (for subatomic particles).
From these fundamental units can be derived many other units. For example, length can be used to derive area (square meters, square centimeters, etc) or volume (cubic meters, cubic centimeters, liters, milliliters, etc).

Density is another derived property. It is a measure of the mass per unit volume. Density relates volume and mass. If you know any two of them, you can always calculate the third because D = m/V.

In your case, you're given the density (0.67 g/ml) and the volume (18 gal) and are asked to calculate the weight in kilograms (which is, strictly speaking, a unit of mass but we'll just let it stand as is).
1. Convert your given volume units (gallons) to units consistent with your given density (ml). 18 gal x 3785.4 ml/gal = 68137.2 ml
2. Solve equation D=m/V for mass: m = D x V
3. Substitute numerical values into #2: (0.67 g/ml x 68137.2 ml = 45651.9 g)
4. Convert the answer in #3 from grams to kilograms: 45651.9 g x (1 kg/1000 g) = 46 kg (using 2 sig fig since volume was given to 2 sig fig)
(Strictly speaking, what we've calculated is the mass of the gasoline. Its weight would actually be found by multiplying that mass by the acceleration of gravity. Unfortunately, many people outside of the physics world get pretty sloppy with the mass / weight distinction.)
4. Music.... I've decided.... You are my new best friend!!! :icon_hug:

I'm going to try and work this through now. I think it was number 1 in your post that was throwing me off. Thank you so much! I'm sure I'll be back for more over the weekend! :trout:

Jen

ETA: I was YELLING at my kids last night (trying to get them to go to bed so I could do this stuff)... I feel really bad today now...
5. Quote from luvmy3kids
Music.... I've decided.... You are my new best friend!!! :icon_hug:

I'm going to try and work this through now. I think it was number 1 in your post that was throwing me off. Thank you so much! I'm sure I'll be back for more over the weekend! :trout:

Jen

ETA: I was YELLING at my kids last night (trying to get them to go to bed so I could do this stuff)... I feel really bad today now...

Hang in there. Give your kids a big hug and apologize. Don't feel guilty because it doesn't help anybody.
6. So I worked through the problem (with your help) and I looked at my answer that I derived last night... I GOT THE RIGHT ANSWER!!

I worked it out a little different than your order... but at least I got the answer right.

I feel a lot better now. Thank you, again, for your help. I appreciate it so much!!

Jen
7. I'm glad it's starting to click.

The order of the steps isn't so important but you just need to be sure to get everything into a common set of units.

Keep at it -- you're getting there.