Let us solve each question step by step, showing the calculations and reasoning:

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Problem 1: Mass of silver in the necklace

Sterling silver contains 93.5% silver, and the necklace weighs 15 g.
We find the mass of silver using:
$\text{Mass of silver} = \text{Percentage of silver} \times \text{Total mass}$ Substituting the given values:
$\text{Mass of silver} = 0.935 \times 15 = 14.025 \, \text{g}$

Correct Answer: d. 14.0 g

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Problem 2: Volume of isopropyl alcohol in 500 mL solution

Rubbing alcohol is 70% isopropyl alcohol. This means 70 mL of isopropyl alcohol is present in every 100 mL of solution.

To find the volume of isopropyl alcohol in 500 mL solution:
$\text{Volume of alcohol} = 0.70 \times 500 = 350 \, \text{mL}$

Correct Answer: b. 350 mL

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Problem 3: Concentration of KCl in the solution

The formula for percent concentration (% $m/v$) is:
$\% (m/v) = \frac{\text{Mass of solute (g)}}{\text{Volume of solution (mL)}} \times 100$ Substituting the given values:
$\% (m/v) = \frac{15.0}{600.0} \times 100 = 2.50\%$

Correct Answer: d. 2.50%

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Problem 4: Volume of 0.250 M KOH solution needed to contain 6.31 g of KOH

First, calculate the number of moles of KOH using its molar mass (KOH = 39.1 + 16 + 1 = 56.1 g/mol):
$\text{Moles of KOH} = \frac{\text{Mass of KOH}}{\text{Molar mass of KOH}} = \frac{6.31}{56.1} \approx 0.1125 \, \text{mol}$

Now, use the molarity equation to find the volume:
$M = \frac{\text{moles of solute}}{\text{volume of solution in liters}} \implies \text{Volume (L)} = \frac{\text{moles}}{M}$ Substituting:
$\text{Volume (L)} = \frac{0.1125}{0.250} = 0.450 \, \text{L} = 450 \, \text{mL}$

Correct Answer: c. 450 mL

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Final Answers:

1. d. 14.0 g
2. b. 350 mL
3. d. 2.50%
4. c. 450 mL

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Would you like detailed explanations or have any questions?
Here are five related problems you might find helpful:

1. If a necklace weighs 20 g, how much silver is present at 93.5% purity?
2. What volume of water is needed to dilute 70 mL of rubbing alcohol to a 35% solution?
3. Find the % concentration (m/v) of 10 g sugar in 200 mL solution.
4. How many grams of KOH are needed to prepare 1 L of a 0.5 M solution?
5. A 15 g alloy contains 80% copper. What is the mass of copper?

Tip: Always verify unit conversions and formulas to avoid errors in calculations.