How do you find moles of ions in these chemistry problems?

Question

How many moles of ions are in the following?

(a) 0.0132 g AgNO₃ 

 mol 

(b) 0.175 mol K₂CrO₄ 

 mol 

(c) 0.481 g Ba(OH)₂ 

 mol 

(d) 1.37  10^-9 mol Na₂CO₃ 

 mol

Answer 1

a) 0.0132 g AgNO3. How many moles of Ag+ and NO3- ions are there.

Find molar mass of AgNO3. It is 169.9 g/mole

Assuming the AgNO3 is in solution, it will ionize, otherwise there are no Ag+ and NO3- ions. But in solution,

you find moles of AgNO3. That is 0.0132 g AgNO3 x 1 mol AgNO3/169.9 g = 7.77x10-5 moles.

Since there is 1 mole Ag and 1 mole NO3 per mole of AgNO3, this is also the number of moles of Ag+ and NO3- ions.

b) 0.175 moles K₂CrO₄

Same idea as above to begin with.

molar mas K₂CrO₄ = 194.2 g/mole

moles K₂CrO₄ = 0.175 g x 1 mol/194.2 g = 9.01x10-4 moles

Since there are 2 moles of K⁺ and 1 mole of CrO₄^2- ions per mole of K₂CrO₄ we have...

moles K⁺ = 2x 9.01x10^-4 moles = 1.80x10^-3 moles K⁺

moles CrO₄^2- = 9.01x10^-4 moles CrO₄^2-

Maybe you can do the others???

Answer 2

To solve use the equation: g= mol/atomic weight to get the molar masses or to solve for moles for each of the substances. You can use the periodic table, add up all the atomic weights and you would solve for the variable that is missing, using dimensional analysis to make units cancel out: For example

a) you have .0132 g of of Silver Nitrate AgNO3.

first step sum up the atomic weights:

Ag (107.9 g/mol) + N (14.01 g/mol) + O (16 x3 g/mol) = 169.91 g/mol

Notice how we multiply Oxygen's atomic weight by 3.

Now use dimensional analysis to get the the units to cancel so that you will be able to solve for what you are looking for. for this problem you will multiply the grams by the inverse or reciprocal of the molar mass getting them to cancel:

0.0132g x 1mol/169.91g = 7.77 x10*-5

the grams cancel out and you have 7.77 x10*-5

the * is a symbol for raising to and exponent when not finding the carrot but or another way is 7.77 e -5.

this is a very small number.

you would use this same approach to solve for each writing and using dimensional analysis to get units to cancel.

Let's look at example b

for this problem you will add up the atomic weights and do a mol ratio to get the proportion for the mol given in the problem.

first step sum up the atomic weights or molar masses:

Kr (83.8x2) + Cr (51.99) + O (16x4) = 283.59 g/mol

now set up the ratio and solve:

if 1 mol = 283.59g/mol

_________________

0.175mol = X Now cross multiply and divide to solve for X

X= 283.59(0.175) = 49.63 g/mol

now if you take the inverse or reciprocal 1/49.63, you will have the moles for the grams proportional to what is given in the problem, so .02015 mol/g