A 2.50 gram sample of bronze was dissolved in sulfuric acid. The copper in the sample reacted with sulfuric acid to form copper sulfate.

Cu (s) +2 H2SO4 (aq) -> CuSO4 (aq) +2 H2O (l)
The copper sulfate formed in this reaction was mixed with I to form copper iodide:
CuSO4 (aq) + 5 I- (aq) -> 2CuI (s) + I3- (aq) + 2 SO4^2- (aq)

The I3- (aq) formed in this reaction was then titrated with S2O3^2- (aq):
I3- (aq) + S2O3^2- (aq) -> 3 I- (aq) + S4O6^2- (aq)

Calculate the mass percent of copper in the original sample if 31.5 mL of 1.00 M S2O3^2- were consumed in this titration.

I haven't taken Gen. Chem. in 2 years and can't remember how to do titration questions. I'm trying to help a freshman that has an exam on Wed., but she doesn't have any notes to help me with this problem.

moles S2O3^-2 = M x L = 0.0315*1M = 0.0315.

Now you want to convert this to moles Cu, using the coefficients in the balanced equation. I have shortened the above sequence into the following
1 mole Cu ==>1 mole CuSO4 ==> 1 mole I3^- ==> 2 moles S2O3^-2; therefore, 1/2 mole Cu = 1 mole S2O3^-2.
Since mole S2O3^-2 = 0.0315, then moles Cu must be 1/2 that.
g Cu = moles Cu x molar mass Cu.
mass %Cu = (mass Cu/mass sample)*100 = ??

80%

To calculate the mass percent of copper in the original sample, we need to use the information given and perform the necessary calculations.

Step 1: Calculate the moles of S2O3^2- used in the titration.
We are given that 31.5 mL of 1.00 M S2O3^2- was consumed in the titration. First, convert mL to L:
31.5 mL = 0.0315 L
Then, use the equation:
moles of S2O3^2- = concentration x volume
moles of S2O3^2- = 1.00 M x 0.0315 L

Step 2: Calculate the moles of I3- reacted in the titration.
From the balanced equation, we know that the reaction between I3- and S2O3^2- is 1:1. Thus, the moles of S2O3^2- consumed will be equal to the moles of I3- reacted.

Step 3: Calculate the moles of copper in the sample.
From the equation:
I3- (aq) + S2O3^2- (aq) -> 3 I- (aq) + S4O6^2- (aq)
We can see that for every mole of I3- reacted, 1 mole of copper (Cu) is present in the original sample.

Step 4: Calculate the mass of copper in the original sample.
Given that the sample was 2.50 grams, and we know the moles of copper in the sample from Step 3, we can use the molar mass of copper (63.55 g/mol) to calculate the mass of copper:
mass of copper = moles of copper x molar mass of copper

Step 5: Calculate the mass percent of copper in the original sample.
The mass percent is calculated by dividing the mass of copper by the mass of the original sample and then multiplying by 100:
mass percent of copper = (mass of copper / mass of original sample) x 100

By following these steps, we can determine the mass percent of copper in the original sample.

To calculate the mass percent of copper in the original sample, we can use the information provided in the question.

Let's break down the problem step by step:

Step 1: Calculate the number of moles of S2O3^2- used in the titration.

Given:
Volume of S2O3^2- solution = 31.5 mL = 0.0315 L
Concentration of S2O3^2- solution = 1.00 M

Using the equation Molarity (M) = Moles (mol) / Volume (L), we can rearrange the equation to solve for moles:
Moles (mol) = Molarity (M) × Volume (L)

Moles of S2O3^2- = 1.00 M × 0.0315 L

Step 2: Calculate the number of moles of I3- reacted in the titration.

From the balanced equation:
1 mole of I3- reacts with 1 mole of S2O3^2-

So, the number of moles of I3- = number of moles of S2O3^2-
Moles of I3- = Moles of S2O3^2-

Step 3: Calculate the number of moles of Cu in the original sample.

From the balanced equation:
2 moles of CuI are formed from 1 mole of Cu

So, the number of moles of Cu = (moles of I3-) / 2

Moles of Cu = (moles of I3-) / 2

Step 4: Calculate the mass of Cu in the original sample.

Given:
Mass of the sample = 2.50 grams

To calculate the mass percent, we need the mass of just the Cu in the sample:
Mass of Cu = Mass of sample × (moles of Cu / moles of sample)

Mass percent of Cu = (Mass of Cu / Mass of sample) × 100

Substituting the values calculated in the previous steps, we can now solve the problem.