Molarity of Copper Ion solution produced from Copper, Nitric Acid, and Water

[torquemada]

Homework Statement

(c) A 0.1036 g sample of copper metal is dissolved in 48 mL of concentrated HNO3 to form Cu2+ ions and then water is added to make a total volume of 208.1 mL. (Calculate the molarity of Cu2+.)

Homework Equations

Molarity = Moles/Liter

The Attempt at a Solution

.1036g/63.55g/mol = .0016302124 moles

.0016302124/.2081 (moles of Copper Ions over final Volume in liters) = .0078337935 M - webassign says this is wrong. I'm thinking perhaps the reaction between copper metal and HNO3 to yield Cu2+ ions isn't 1:1 in stoichiometry and maybe i have to balance the equation and use oxidation states to get it right but i have no idea how to approach balancing this reaction because I'm not even clear what the products are and then I'm not sure if water does anything additional to the solution besides add volume, maybe some other reaction happens? thx

The molarity they want is the molarity of Cu2+

 

[torquemada]

nvm - my answer is correct but webassign was being retarded and inconsistent with significant figures, as it often is. thanks anyway

 

[symbolipoint]

The balancing is not necessary. Copper metal, all converted to Cu⁺².
How many moles of Cu? How much volume of final solution as liters? Calculate moles Cu in 1 liter. That is your molarity of Cu (in your example, as Cu⁺² ions.)

Is your webassign expecting correct accuracy for the given values? You showed too many significant figures in your work.

(Just finished writing my message while torquemada's post was made.)

 

[torquemada]

oh ok hehe

 

[DeeNos]

ions, not the molarity of the copper metal. So you need to calculate how many moles of Cu2+ ions are present in the final solution, and then divide by the total volume.

First, calculate the moles of Cu2+ ions produced from the 0.1036 g of copper metal:

0.1036 g Cu * (1 mol Cu / 63.55 g Cu) * (2 mol Cu2+ / 1 mol Cu) = 0.003259 mol Cu2+

Then, calculate the final volume in liters:

208.1 mL * (1 L / 1000 mL) = 0.2081 L

Now you can calculate the molarity of Cu2+ ions:

0.003259 mol Cu2+ / 0.2081 L = 0.01566 M Cu2+