[Portomar1970]

I am really struggling with these 3 questions, can anyone help or shed some light? Thanks everyone for the much needed help.


In this investigation you will determine the density of distilled water by finding the density of 3 separate samples and calculating an average. You will then repeat this procedure for an unknown liquid. Then you will compare your measured values with the accepted values for the densities of these liquids.

Why is the precision of the laboratory balance (how many decimal places) that you use in this investigation important? What effect would a less precise balance have on your results?

What is the advantage of doing three trials of measurements instead of just one when determining the density of each liquid?

Why should you avoid skin contact with the unknown solution?

[Borek]

To get help you need to show attempts at answering the problem, this is a forum policy.

[Portomar1970]

Ok here is what I came up with:

The precision of the balance is important because the more precise the scale is, the accuracy of the values obtained compared to the accepted values for the densities of these liquids may increase. A less precise balance could take away the accuracy of your results. In addition, an inconsistent number of significant figures wil ultimately alter the accuracy of the result.

The advantage of doing three trials is that it increases the probability of getting closer to the actual accepted values for the densities of these liquids. Also by conducting 3 trials, you can also look at the precision of your obtained densities. Sometimes, when measuring a substance, other factors may intervene and prevent you from getting the stand result. Therefore, by doing 3 trials, it increases the chances of getting a closer, more accurate result.

[Borek]

The precision of the balance is important because the more precise the scale is, the accuracy of the values obtained compared to the accepted values for the densities of these liquids may increase.

You are mixing precision and accuracy in one statement in a way I don't like. They have both a separate, well defined meaning. Chances are you were not taught them yet, but it still doesn't make the statement correct.

In addition, an inconsistent number of significant figures wil ultimately alter the accuracy of the result.

Not sure what you mean by that.

The advantage of doing three trials is that it increases the probability of getting closer to the actual accepted values for the densities of these liquids. Also by conducting 3 trials, you can also look at the precision of your obtained densities. Sometimes, when measuring a substance, other factors may intervene and prevent you from getting the stand result. Therefore, by doing 3 trials, it increases the chances of getting a closer, more accurate result.

More or less correct, but such discussion without mentioning experimental error will be never complete. Imagine taking one measurement and getting 1.0 g/mL. Imagine getting three measurements and getting results of 1.0, 2.0 and 2.0 g/mL. What does the second approach tells you about the first measurement?

[Enthalpy]

Better scales can't harm, but in the experiments I did, it was not the weakest element.

Imagine banal scales give 0.1g accuracy in a 100g measure (laboratory scales are much better). That's already 0.1%.

The weight of the displaced air is more than 0.1% the weight of the water. Forgetting it would be worse than using banal scales.

Water expands by 200ppm/K around room temperature so 5K inaccuracy brings 0.1% too. Nothing tragic, but keep an eye on it.

And above all, how accurate is the container? Imagine 100mL as D=50mm h=50mm approximately. 0.1% demands 17µm accuracy on both dimensions.

I had turned my containers of aluminium, and then the dimensions are easily that accurate. But what if the bottle is of plastic or glass? And in the case of plastic, thermal expansion is a worry too.

Then, what defines the liquid's filling level? If a D=50mm cavity ends straight you won't estimate the filling level to 17µm. It needs to observe the stop level in a narrow neck, but then the container's shape imposes to assemble two parts, which may ruin the manufacturing's accuracy.

No wonder that, everytime possible, experimenters only compare densities of liquids, often referring to water.

If an absolute density measure is mandatory, the standard method is to plunge a metal cylinder of accurate diameter in the unknown liquid, move it down by an accurate height, and measure how much additional mass of the unknown liquid has overflown of the containing beaker. Because of capillarity, the plunger must be uniformy clean and move always down.