Question

CHE115 Name: Tear this page out of your laboratory manual and submit it to your TA with your answers B. Post-Lab Assessment Questions: (25 pts) The post-lab assessment questions for each laboratory experiment are questions that you must complete at home and hand in to your laboratory instructor the following week These questions are designed to stimulate thought and may not always be simple to answer. You should always answer them as thoroughly as you are able to 1. (5 pts) People often think that heavy items will sink in water, while light things will float. Is this always true? Explain your thinking 2 (6 pts) In chemistry, chemists often use trends in their data to help them make predictions about future experiments. Using the trends that you observed in the lab today, can you predict where the following 2 items would fit if they were added to your column? Item to consider A piece of pure gold A piece of polypropylene plastic Volume (mL) Mass (g) 14.048 2.27 0.728 2.5 3. (4 pts) A perfect cube of aluminum metal was found to have a mass of 16.2 g. If the density of aluminum is 2.7 g/cm3, what is the length of each side of the aluminum cube? 4. (5 pts) Keeping in mind what you did in the lab this week, do you think that determining the volumes of your solid samples (rubber stopper, crock stopper and pennies) by measuring their dimensions and calculating it mathematically would be more or less accurate than determining their volumes by water displacement? Explain. 5. (5 pts) Do you think that the water displacement method will work for calculating the density of a tablespoon of sugar? Explain.

Answer 1

1. The statement is not true.

An object floats when the weight force of the object is equal to the upward force exerted by water on the object. The force exerted by water on the object is more when volume of the object under water increases.

2. Density of pure gold object is: 14.048 g / 0.728 mL = 19.3 g / mL

The density of pure gold ( Literature value) matches with the given data. So, it fits.

Density of polypropylene plastic is: 2.27 g / 2.5 mL = 0.908 g / mL

The density of polypropylene plastic ( Literature value is 0.946 g / mL) does not match with the given data. So, it does not fit.

3. Volume of the aluminium cube is: 16.2 g / ( 2.7 g/ cm³) = 6 cm3

If l be the length of the cube, l³ = 6 cm³

So, l = 1.82 cm

4. Determining volume by water displacement is more accurate as determining the dimensions would result in (a) errors in mathematical calculations, (b) human error in measuring.