Chemical Equation Help: ASAP! Please.

[CreationArtist]

Please be kind enough to help out.
I really need this help ASAP.

HBr + Mg(OH)₂ -> H₂O + NaCl

H₂S + Ba(OH)₂ -> H₂O + NaCl


I don't know if these are correct/balanced.
Can you help me out ASAP?

Thanks!

Also, how do you do this problem?

4AU + 8KCN + O₂ + 2H₂O -> 4KAu(CN)₂ + 4KOH

How many moles of KCN are required to extract 29.0 g pf gold (about an ounce) of gold?


Also, the other day I could not remember how to do this type of problem:

Nitrogen oxide reacts instantly with oxygen gas to form nitrogen dioxide. Write the equation. In one experiement, 0.866 moles of NO is mixed with 0.503 moles of oxygen gas. Calculate which of the reactants is the limiting one? How many moles of nitrogen dioxide are produced?

or this simple one that I simply forgot how to use 6.02EE₂₃.
The carat is the unit of mass used by jewelers. One carat is exactly 200 mg. How many carbon atoms are present in a 24-carat diamond?


If you could walk me through each problem, that would be greatly appreciated. Thanks.

And sorry for all of the questions (my midterm is coming up and I have a lot of studying.. we don't use that great of a book either) and I really need to know how to do all of these very shortly.

[chempass]

Please be kind enough to help out.
I really need this help ASAP.

HBr + Mg(OH)₂ -> H₂O + NaCl

H₂S + Ba(OH)₂ -> H₂O + NaCl

I don't know if these are correct/balanced.
Can you help me out ASAP?

Thanks!

They are both not correct.
Here are balanced and correct equations:
2HBr + Mg(OH)₂ = MgBr₂ + 2H₂O
H₂S + Ba(OH)₂ = BaS + 2H₂O

Also, how do you do this problem?

4Au + 8KCN + O₂ + 2H₂O -> 4KAu(CN)₂ + 4KOH

How many moles of KCN are required to extract 29.0 g pf gold (about an ounce) of gold?

m(Au) = 29.0 g (given)
M(Au) = 196.97 (atomic weight of Au: http://www.webqc.org/periodictable-Gold-Au.html)
n(Au) = m(Au) / M(Au) (calculate how many moles of Au are in 29.0 g)
n(KCN) = (n(Au) / 4) * 8 (calculate how many moles of KCN are required)

Also, the other day I could not remember how to do this type of problem:

Nitrogen oxide reacts instantly with oxygen gas to form nitrogen dioxide. Write the equation. In one experiement, 0.866 moles of NO is mixed with 0.503 moles of oxygen gas. Calculate which of the reactants is the limiting one? How many moles of nitrogen dioxide are produced?

2NO + O₂ = 2NO₂

0.866/2 < 0.503 therefore NO is limiting
n(NO₂) = n(NO) * 2 /2 = 0.866

or this simple one that I simply forgot how to use 6.02EE₂₃.
The carat is the unit of mass used by jewelers. One carat is exactly 200 mg. How many carbon atoms are present in a 24-carat diamond?

m(C) = 24 * 200 mg (calculate the mass of carbon)
n(C) = m(C) / M(C) (calculate number of moles of carbon)
N(C) = n(C) * 6.02E₂₃ (calculate number of carbon atoms)

[CreationArtist]

Thank you so much, I PM'd you back.

[chempass]

Just making sure I understood the two that you didn't make the calculations on.. I got:

.0046 moles of KCN

and for the other I got:

2.4EE₂₆ Atoms

1) 29.0 / 196.97 * 2 =0.294 moles of KCN

2) 24 * (200 / 1000) / 12.01 * 6.02E₂₃ = 2.419E+23 atoms

And sorry for all of the hassle (it's much appreciated), but it would really help if you could quickly show me how to do this:

Platinum forms two different compounds with chlorine. One contains 26.7% Cl by mass. The other contains 42.1% Cl by mass. Determine the empirical formulae of the two compounds.

100 - 26.7= 73.3

26.7% of Cl (MM=35.453)
73.3% of Pt (MM=195.08)

Cl
26.7g. x 1 mole/35.453 = 0.753 moles

Pt
73.3g. x 1 mole/195.08 = 0.376 moles

0.753/0.376 = 2

0.376/0.376 = 1

2Cl : 1Pt ratio, therefore the first compound is:

PtCl₂

and you do the same thing for the other compound?

Yes, you need to do the same thing. (you will get PtCl₄ )

It's better to post here rather than PM.