Chemistry questions

Deleted User · Sep 20, 2017

My friend has messaged me asking me to help him with his chemistry. I can do most of the questions myself but some of them I never learned how to do. This is where you guys come in.

A polymer molecule has a mass of 2.5 X 10-20g. Determine the molar mass of the polymer.

I gave it a shot and substituted n = 1 and m = 2.5 X 10-20g and put it into the formula M=m/n but that left me with a value of M as 2.5 X 10-20g. This is a multiple choice question and my answer did not appear in the list of possible answers.

EDIT: Is there any way to put "-20" in superscript?

AbyssalMonkey · Sep 20, 2017

avagadro's number is the amount of atoms in one MOLE (molar)

Multiply by it to get the amount of mass in one mole (Molar mass)

Avagadro's number is 6.0221409e+23, so your answer would be 15000 grams/mol

Deleted User · Sep 20, 2017

What volume of a 0.5 mol dm^-3 solution of sodium hydroxide can be prepared from 2g of the solid?

I have no idea. I never learned what dm^-3 is. Looking at my old textbook I can't find this in their the solutions chapter or the acids and bases chapter.

AbyssalMonkey said:
avagadro's number is the amount of atoms in one MOLE (molar)

Multiply by it to get the amount of mass in one mole (Molar mass)

Avagadro's number is 6.0221409e+23, so your answer would be 15000 grams/mol

Thank you. Haven't done chemistry for years now. My skills have gotten rusty.

AbyssalMonkey · Sep 20, 2017

sodium hydroxide has a molar mass of 39.997. dm is decimeter, or 1/10 of a meter, therefore dm cubed is a liter using the definition of centimeter cubed = 1 milliliter.

.5 mol dm^-3 is asking for a half molar solution, so .5 moles/liter of solution.

(2g) / (39.997 g/mol) gives ~1/20 mol of sodium hydroxide. (1/20 mol) / (.5 mol/liter) = 1/10 liters, or 100mL

Deleted User · Sep 21, 2017

Calculate how many moles of hydrochloric acid are present in 0.80dm^3 of a solution with a concentration of 0.40 mol dm^-3.

Using the information in the post above I calculated it to be 0.32. But I'm not familiar with the unit dm^-3 so I will ask just to be safe.

What volume would 3.20g of sulphur dioxide occupy at STP?

The answer I got by calculating n(SO2) and the putting it into the equation pV = nRT is 1239.41. It's a multiple choice question and the answers provided aren't even close to this.

AbyssalMonkey said:
sodium hydroxide has a molar mass of 39.997. dm is decimeter, or 1/10 of a meter, therefore dm cubed is a liter using the definition of centimeter cubed = 1 milliliter.

.5 mol dm^-3 is asking for a half molar solution, so .5 moles/liter of solution.

(2g) / (39.997 g/mol) gives ~1/20 mol of sodium hydroxide. (1/20 mol) / (.5 mol/liter) = 1/10 liters, or 100mL

Thank you. Again.

AbyssalMonkey · Sep 21, 2017

First answer looks right.

STP is standard temperature (0C, or 273.15K) and pressure (100000 pascals). n = 3.20 mol of SO2. R = 8.3144598 (m^3 * Pa)/(K*mol)

Dividing Pressure over gives V = (3.2[mol] * 8.3144[R] * 273.15[T])/(100000

) = 7.267 x 10^2 m^3 (0.073 m^3), or maybe 73 liters.

Deleted User · Sep 21, 2017

AbyssalMonkey said:
First answer looks right.

STP is standard temperature (0C, or 273.15K) and pressure (100000 pascals). n = 3.20 mol of SO2. R = 8.3144598 (m^3 * Pa)/(K*mol)

Dividing Pressure over gives V = (3.2[mol] * 8.3144[R] * 273.15[T])/(100000
) = 7.267 x 10^2 m^3 (0.073 m^3), or maybe 73 liters.

My mistake is obvious. I did the calculations using SLC instead of STP. The answers provided on the paper are:

A. 0.14 dm^3
B. 0.28 dm^3
C. 1.12 dm^3
D. 2.24 dm^3

AbyssalMonkey · Sep 21, 2017

Whoops, I'm silly, I read that as 3.20 mol of SO2 :hateit:

Recalculation time. .05 mol.

The answer I got is 1.13 liters (dm^3) with 100kPa, the standard IUPAC definition but got a more consistent 1.1207 liters at 1 atm. Figure they use old standards. :gun:

Deleted User · Sep 21, 2017

AbyssalMonkey said:
Whoops, I'm silly, I read that as 3.20 mol of SO2

Recalculation time. .05 mol.

The answer I got is 1.13 liters (dm^3) with 100kPa, the standard IUPAC definition but got a more consistent 1.1207 liters at 1 atm. Figure they use old standards.

And that answers my last question. You've helped a lot.

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