Principle Chemistry Questions?

So what do you think you should do next? (If you were hoping to get the answer I am afraid you are on the wrong site.)

So start the process…

Oh, I’m not looking for answers, that would be cheating. I am simply looking for aid.

I answered the second equation and I think I figured it out (plugged in the numbers and found n and got 1.21 etc.). However I’m not sure how to figure out the first problem… starter tips?

You want your numbers to be in the same unit of measurement…that’s your starter tip.

would I take the dimensions (15×24) and multiply the width and length respectively by 2.54 centimeters? I.E. 15(2.54) x 25.4(2.54) = a really big number (2458.1). I don’t think that is right, so what am I missing?

@GreenFinch_YellowCanary You went about that just right, actually. :]

You have inches and centimeters, the first thing you need to do is get them both into the same unit of measurement. Since your final answer is to be in cm^2, I recommend converting the inches into centimeters. Then going from there.

@Seaofclouds
I did convert them to centimeters. 15 in = 38.1 cm and 25.4 in = 64.5 cm
HOWEVER when I go to find the area of the rectangle (which if i’m not mistaken is A=LW)
I get a huge number which doesn’t apply to ANY of my options.

The 25.4 was already in cm in your original question. No need to change it.

“the 25.4 was already in cm in your original question. No need to change it.”

Eek, I didn’t notice that! Okay, you didn’t do it quite right the first time. Sorry, haha

Oh my gosh, I can’t believe I missed that! Thank you!

Do either of you know the equation for equating mg to M (moles/Liter)?

It depends on the molecular weight of the substance you are converting. Generally you change the mg to grams, then the grams you have/the molecular weight (g/mol) which will give you the mol.

For example: 39.8 grams of ammonia (NH3) (already put into grams from mg)
Ammonia weighs 17 grams per mole. Therefore, 39.8 g / (17 g/mol) = 2.34 mol

Now I feel like I’m in Chemistry class again.

This was my question:

A sample of 14.6 mg of MgCl2 was dissolved in water to make 15.0 mL of solution. The resulting concentration of Chloride Ion (Cl– ) in moles/liter was:

So I have moles of 0.0147. Then how do I incorporate the 15 mL?

Is it: 0.00511 M ? Since I only need a part (the Cl-) ?

The 15mL is how much water was used to make up the solution. Once you figure out how many moles you have, it will be x moles/15mL. To get moles/L, you will have to convert the mL to L.

There are 1000 mg in a Gram.
So 14.6 mg = 0.0146 grams yes?

If that is the case, I take the grams, 0.0146 and divide by g/mol of MgCl2 (95.211) to get 1.53×10–4 moles. Yes?

If that is so, I take that moles and divide by 0.015L (since there are 1000 mL in a L) to get 0.0102?

Then what about the Cl-?

Sounds right to me. Try running the numbers backwards just to be sure it works going in reverse too (that’s one of the best ways to be sure conversions are correct).

Thank you so much for all your help! (and not giving me the answers but making sure I figure it out). Another question though, what is the equation that would help me solve this? I know there is one. I think it has to do with something PV=nRT.

Torr is pressure right? But I do not have R. I use to know what the moles/torr was but I don’t remember so I don’t know what the equation for this would be….

A balloon filled with 500 L of Helium at 20ºC and 750 torr was allowed to rise to an altitude at which the pressure had dropped to 600 torr and the temperature was -33ºC. The volume occupied by the Helium in the balloon was now:

Wait… would I just use PV = RT ?
In which I find R in the first half and then use it to find V in the second?
If so, wouldn’t I need to change celsius to Kelvin? Would that then be 293.15K?

@GreenFinch_YellowCanary Torr is indeed pressure, and R is a constant so it always has a value of 0.08206 L·Atm/K·mol, BUT you have to make sure when using PV=nRT with that value of R that your units match, meaning that pressure must be in atmospheres (so you’d have to convert from torr), temperature must be in Kelvin (convert from Celsius) and volume must be in liters (you’re good). Personally, in this situation, I would use PV/T=PV/T where you set your initial conditions equal to the final conditions. When you use this formula, you don’t have to use any particular units as long as you keep your units consistent throughout the problem. Except that you’ll need to convert from Celsius to Kelvin, since a negative temperature doesn’t really make sense in the context of this formula. This conversion requires that you add 273.15 to your Celsius temperature.

so I go 750 divided by 760 torr/atm? to get 0.9868

Cross multiply and solve for x, with x being the atm = to 750 torr.

So 1atm/760torr = xatm/750torr

Alright I set up the equation and crossed multiplied and got 652.69 HOWEVER it isn’t one of my options. I went:

0.98684(500L) divided by 293.15 = 0.7895V divided by 306.15

My teacher always said, let your units do the work for you.

750 torr x (1 atm/760 torr)

See what I’m doing here: The torrs cancel, since you have a label of torrs on the top and on the bottom. This leaves you with a label of atmospheres, so you know you’ve done it right. Also, that fraction is actually equal to one, since 1 atm = 760 torrs, so you know that you haven’t actually altered your value – you’ve just changed the units.

NEVERMIND, I got it. I did 33 C instead of NEGATIVE 33… big difference

THANK YOU SO MUCH FOR ALL OF YOUR HELP!!! Now I get to go study for my Heterotroph Midterm for tomorrow! :)