## HW Fundamentals E15- what is 34.02 grams?

Emily Vainberg 1D
Posts: 102
Joined: Sat Jul 20, 2019 12:15 am

### HW Fundamentals E15- what is 34.02 grams?

For Problem E15 in the Fundamentals section, the problem asks to find the molar mass of the metal. The solution says they subtracted 34.02 from 74.20 but where does the 34.02g.mol come from? I understand that once they got 40.08 g.mol for the answer you can conclude the molecule is Calcium, but I am having trouble understanding how they got the 34.02.

Tracy Tolentino_2E
Posts: 140
Joined: Sat Sep 07, 2019 12:17 am

### Re: HW Fundamentals E15- what is 34.02 grams?

34.02 g is the molar mass of the (OH)2 portion of M(OH)2. By subtracting 34.02 g from 74.10 g, we know the molar mass of the metal, and can figure out what the metal is.

Posts: 51
Joined: Sat Aug 24, 2019 12:16 am

### Re: HW Fundamentals E15- what is 34.02 grams?

To find the metal in M(OH)s, you should subtract the molar mass of (OH)2 from the molar mass of the whole molecule.

The mass of (OH)2 = (15.999 g/mol + 1.008 g/mol) * 2 = 34.014 g/mol.

Then to find the metal: 74.10 g/mol - 34.014 g/mol = 40.09 g/mol, which is closest to the molar mass of Ca.

MMckinney_4H
Posts: 61
Joined: Sat Sep 28, 2019 12:16 am

### Re: HW Fundamentals E15- what is 34.02 grams?

The "M" in M(OH)(sub)2 refers to an unknown metal which you have to find based off of knowing that the two Oxygens and two hydrogens would account for 34 grams of the 74 gram molar mass of the metal hydroxide. If you subtract the 34 from 74, you find that the remaining 40 grams must be the molar mass of the unknown metal which must be Ca. From there, you recognize that Calcium Sulfide (the sulfide of the unknown metal) has a molar mass of 72.138 grams/mol by adding the molar mass of the S to that of Ca.