[Rookie]

I did a lab today in which I had to float an egg in a solution of water and sodium chloride. Then the teacher asked how valid is the assumption that aqueous sodium chloride is an adequate replacement for aqueous sodium hydroxide in this experiment? and to explain my reasoning. The way I thought about it is that both Chloride and hydroxide have a negative charge and obviously sodium has a positive, therefore since both of the elements cancel each other out, then that's enough reason to assume that aqueous sodium chloride is a good replacement of sodium hydroxide .., I somehow think that i am wrong though... 

[Donaldson Tan]

It has nothing to do with the charges.

The floatation of the egg depends on the desnity of liquid medium. Adding salt increases the density of water, and thus the buoyancy force. The buoyancy force is equal in magnitude of the weight of the liquid medium displaced.

The same mass of NaCl must be added in order for NaCl to be a good replacement of the NaOH. This will ensure that the liquid medium will have the same density, regardless the salt added was NaOH or NaCl.

If you must consider the chemical properties, the replacement salt should be inert.

[Borek]

The same mass of NaCl must be added in order for NaCl to be a good replacement of the NaOH. This will ensure that the liquid medium will have the same density, regardless the salt added was NaOH or NaCl.

No such dependence Geo.

1M NaCl has a density of 1.0386, 1M NaOH 1.0425.

5% NaCl has a density of 1.0340, 5% NaOH 1.0554.

[Mitch]

So what was the result of the experiment???

[AWK]

This will ensure that the liquid medium will have the same density, regardless the salt added was NaOH or NaCl.

NaOH is a salt?

[Borek]

NaOH is a salt?

Skrót my?lowy

[Donaldson Tan]

No such dependence Geo.

1M NaCl has a density of 1.0386, 1M NaOH 1.0425.

5% NaCl has a density of 1.0340, 5% NaOH 1.0554.

I am talking in terms of grams, not moles.

Imagine adding 200g of NaCl or NaOH to 1L of water. Either salt produces the same density.

A salt is formed when the hydrogen ion of an acid is being replaced by a metal cation. Water can act as an acid, so why can't NaOH be considered as a salt?

[Borek]

No such dependence Geo.

1M NaCl has a density of 1.0386, 1M NaOH 1.0425.

5% NaCl has a density of 1.0340, 5% NaOH 1.0554.

I am talking in terms of grams, not moles.

Imagine adding 200g of NaCl or NaOH to 1L of water. Either salt produces the same density.

Take a look at the second line of the data.

[Donaldson Tan]

Imagine adding 200g of NaCl or NaOH to 1L of water. Either salt produces the same density.

5% NaCl has a density of 1.0340, 5% NaOH 1.0554.

I assumed that the volume of the mixture remains constant. It is a fair assumption, which was validated by your data. The discrepancy is merely 2%

[Borek]

I assumed that the volume of the mixture remains constant. It is a fair assumption, which was validated by your data. The discrepancy is merely 2%

It is wrong assumption, INvalidated by the data.

For 20% solutiuons (which you have suggested) discrepancy is much higher:

20% NaCl 1.1478
20% NaOH 1.2219

Note that you are doing two mistakes at the same time. First one is assumption that volume remains constant, second is the way you are calculating difference in densities. You should not look at the difference in densities as initial density (identical in both cases) dominates and flattens the result. You should look at difference in density changes - and then you will see that it is huge.

[Donaldson Tan]

Hence, in order to answer the threadstarter's question, the threadstarter must take in account of volume changes, which is not necessary negligible if the salt concentration is high. Moreover, different salt produces different change in the liquid volume.