GED Science : Solutions

Study concepts, example questions & explanations for GED Science

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Example Questions

Example Question #12 : Ged Science

A student is asked to make \(\displaystyle \small 4L\) of \(\displaystyle \small 0.125M\) NaCl solution. The student is given a stock solution of \(\displaystyle \small 5M\) NaCl. What volumes of stock NaCl solution and pure water could be used to make the desired final solution?

Possible Answers:

\(\displaystyle 100 mL\ \text{stock NaCl and}\ 3900mL\ \text{water}\)

\(\displaystyle 125 mL\ \text{stock NaCl and}\ 4000mL\ \text{water}\)

\(\displaystyle 125 mL\ \text{stock NaCl and}\ 3875mL\ \text{water}\)

\(\displaystyle 100 mL\ \text{stock NaCl and}\ 4000mL\ \text{water}\)

Correct answer:

\(\displaystyle 100 mL\ \text{stock NaCl and}\ 3900mL\ \text{water}\)

Explanation:

Convert liters to mL:

\(\displaystyle \small \frac{4L}{1}=\frac{1000mL}{1L}=4000 mL\)

This is the TOTAL volume of the solution. Next, determine how much NaCl to add.

\(\displaystyle \small c_1v_1=c_2v_2\)

\(\displaystyle \small c_1=5.0M\)

\(\displaystyle \small c_2=0.125M\)

\(\displaystyle \small v_2=4000mL\)

\(\displaystyle \small (5.0M)v_1=(0.125M)(4000mL)\)

\(\displaystyle \small (5.0M)v_1=500MmL\)

\(\displaystyle \small \frac{(5.0M)v_1}{5.0M}=\frac{500MmL}{5.0M}\)

\(\displaystyle \small v_1=100mL\)

Therefore, \(\displaystyle \small 100mL\) of stock NaCl must be added. Subtract this volume from \(\displaystyle \small 4000 mL\) to determine the amount of water used in the final solution.

\(\displaystyle \small 4000mL-100mL=3900mL\ water\)

Example Question #13 : Ged Science

6g of sugar is dissolved in 250mL of water. What is the percent concentration of sugar in mass per unit volume?

Possible Answers:

\(\displaystyle \small 2.4\%\)

\(\displaystyle \small 60\%\)

\(\displaystyle \small 6\%\)

\(\displaystyle \small 0.024\%\)

Correct answer:

\(\displaystyle \small 2.4\%\)

Explanation:

Concentration can be measured in a variety of ways. To find the percent mass per unit volume, we must divide the mass of solute by the volume of solvent and multiply by 100%.

\(\displaystyle C=\frac{m}{V}\times 100\%\)

We know our mass is 6g and our volume is 250mL. Use these values in the equation to find the concentration.

\(\displaystyle C=\frac{6g}{250mL}\times100\%\)

\(\displaystyle C=0.024\times100\%=2.4\%\)

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