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### Problem F.7

Posted: **Thu Oct 04, 2018 10:43 am**

by **Germar G 4F**

A metal M forms an oxide with the formula M2O, for which the mass percentage of the metal is 88.8%. (a) What is the molar mass of the metal? (b) Write the name of the compound.

How would we go about to solve part a) of this problem? So far, I understand that the ratio of the mystery metal "M" to O is 2:1 because of the formula M20. How can I use the given info, 88.8% of M, and this ratio to calculate the amount of molar mass?

### Re: HW Problem F.7

Posted: **Thu Oct 04, 2018 11:33 am**

by **RachaelKoh3A**

Let Mr of M be x.

% composition = 88.8%

Form an equation in x using the formula, % composition = (mass of M / mass of M2O) x 100%.

[2x/(2x+16.00)] x 100% = 88.8%

Then use algebra to solve for x.

2x/(2x+16.00) = 0.888

2x = 1.776x + 14.208

0.224x = 14.208

Eventually you will get x = 63.4 g/mol

Mr of M = 63.4 g/mol

Mr of MO2 = 2(63.4) + 16.00 = 142.8 g/mol

### Re: HW Problem F.7

Posted: **Thu Oct 04, 2018 11:43 am**

by **rikolivares**

For a, you would first need to find the molar mass of two oxygen atoms. It is given that the metal M is 88.8% of the compound, so O2 must be the rest. I think next you can divide percentage of O2 (as a decimal, e.g. 20% would be .20) by its own molar mass, and finally multiply the number by 100 to find the total molar mass of the compound. Then find the molar mass of M in relation to this total molar mass of the compound.

### Re: HW Problem F.7

Posted: **Thu Oct 04, 2018 11:52 am**

by **julia_lok_2K**

I didn't see this in the list of homework problems?

Anyway, how I did it was that since they give you the mass percent of M, you can figure out the mass percent of O by subtracting 88.8 from 100 to get 11.2. You can then set this number out of 100g to get 11.2 g, then find the number of moles of O, which is around 0.7.

Once you get this number, you now know that however many moles of M that you have would be double the number of moles of O (from the 2:1 ratio), so around 1.4 moles. You then use this number to set up an equation finding the molar mass of M. The equation I used was 88.8g * (1 mol / x g) = 1.4 mol, kind of like how you would solve for the number of moles in an empirical formula problem. What you solve for is closest to the molar mass of Cu.

Now that you know what M is, you can then say that the name of the compound is copper(I) oxide and that the molar mass is 63.4 g/mol.

I know I went out of order, but I hope this helps!

### Re: HW Problem F.7

Posted: **Thu Oct 04, 2018 3:34 pm**

by **Germar G 4F**

Sorry guys, just realized this problem isn't even on the HW list. Wow lol. Anyway, thanks for the help!!