Re: What kind of metal with a calculated density of 9.1g/cm3 be?

Hello Allison,

Don't be confused or discouraged! You have learned a very important part of doing a science experiment, which is to know about experimental errors. Let's take a look at your experiment. First, you had to measure the mass of your metal, for which you used a balance. You measured this quite accurately as 158.5 g. I don't know how many decimal places your balance has, but if it was only one then really, the true mass of your metal may have been almost half a gram higher or lower. But this is only a small error in your experiment - calculate your density again using either 159 or 158 g and you will see there is very little difference.

Next, you had to calculate the volume of your metal. Now, if it had a regular shape like a sphere or a cube, this would be easy as you could use a formula. But you say that you used displacement, so your metal was not a regular shape and you had to place it in water and measure the rise in the water level. Your volume was measured as 17.5 cm3, so it sounds as though you could measure to an accuracy of 0.5 cm3. So your volume may have been as little as 17 cm3. Divide 159 by 17 and you see that you get 9.4 g/cm3.

Now, this is still some way from the answer of of 11.3, but you see how small changes have made quite a big difference. Imagine if you were not using a very accurate balance, perhaps one that wasn't calibrated properly. It could be wrong by maybe 5 grams. And it isn't very easy to estimate volumes by displacement unless you have a very accurate measuring cylinder. You might be out by as much as 2 or 3 cm3. So divide 165 g by 15 cm3 and see what your answer could be.

There is another possibility of course - you were told that your metal sample was pure lead, but are you sure that it was? Perhaps it had formed some lead oxide on the surface, perhaps there was air trapped inside that made it seem less dense. Can you think of other ways to determine if your sample was lead?

I hope you can see that when you do an experiment, there are many things to consider: how precise and accurate your measurements are, how good your equipment is, how skilled you are as an observer and how sure you are of your materials. Estimating errors is a very important skill in science - we use statistics to do this, so make sure to learn all about those in your courses.

Good luck and I hope you keep enjoying your experiments,
Neil