Question

You must read the experiment and then answer these questions. 1. Look at the procedure for using a pipet. Why should you not force the last remaining amount of liquid out of the pipet? 2. If you have a 50.00 mL volumetric flask (a flask marked at a specific volume) and a pipette that measures up to 10-mL with marks to the nearest 1/10 mL, how much solute would you add to the flask to make 50.00 mL of a 5.00% solution? 3. The 5.00 % solution needs to be diluted to make 10.00 mL of a 2.00 % solution. What volume of 5.00 % solution should be used? Calculate What % corresponds to the following fractions? 4. % а. % b. 1 C. Your pipet reads as the picture to the right. How should you record your answer? 5. CHEM10 Laboratory Manual/Lab 20- Rancho & Chino Campus 233

Answer 1

1)

When pipets are calibrated, generally they are done so assuming that a certain amount of liquid will remain inside the pipet due to the surface tension. Thus, if we blow out the liquid, the actual volume delivered will be more than the recorded volume and hence, there can be inaccuracies.

2)

We need to make 5% solution having 50.00 mL volume. Thus, volume of solute will be,

Volume = 5% of 50.00 mL = 5*50.00/ 100 = 2.5 mL

Thus, we need to take 2.5 mL of the solute to make 5% solution.

Note that we have used only 1 decimal place because the pipet can measure only up to 1/10th of a mL

3)

We need to convert the 5% (0.05) solution to a 2% (0.02) solution having 10.00 mL total volume

Volume of Solute in 2% solution = 2 % of 10.00 mL 0.02 * 10.00 mL= 0.2 mL

Volume of Solution needed = 0.2 mL / 0.05 = 4.0 mL

Alternatively, we can use the formula,

CiVi = CV

Here, C₁ = Initial concentration = 5%. C₂ = Final Concentration = 2%

Initial Volume = V₁ which we need to find out. Volume Final, V₂ = 10.00 mL.

Thus, we will have,

(5%) (V1) = (2%)(10.00mL) =V1 = x 10.00mL = 4.0mL

Note that we have used only 1 decimal place because the pipet can measure only up to 1/10th of a mL

4)

To find the percent from a fraction, we need to multiply it by 100

= x 100% = 20%

x 100% = 50%

35 35 50 50 X 100% = 70%

5)

For a colorless liquid which forms concave meniscus when looked from top, we always take the reading of the lower meniscus which is 1.4 mL in this case.