Reef Chemistry Question of the Day #40 Water weight

[beaslbob]

again

 

[nrbelk]

I'm going to go with C.

I'm mainly basing this on that one gallon of freshwater is ~8lbs but one gallon of salt water is ~10lbs. The closest answer, at least as I think it in my head is ~26%.

 

[Randy Holmes-Farley]

Hopefully he's throwing a curve here.

No, it wasn't a red herring.

 

[chrisfraser05]

Feel free to delete this one Randy......


Guys, don't try to over think it.

Salinity in ppt is measuring ONLY the additional salts.
We need to look at the weight, or density.
So we need to consider the SG or specific gravity.

The sg of RO water is 1.000
The sg at a the average reef salinity is x.xxx

How much more is x.xxx than 1.000?

 

[chrisfraser05]

Also this is where working in lt and kg makes life so much easier ;-)

 

[reef lover]

i say c 26%

 

[Velodog2]

No, it wasn't a red herring.

oooooh where can I get a red herring? How big a tank should I have ? What do they eat? Are they reef safe?

Answer D.

 

[reggaedrummin]

It's D for me!

 

[Randy Holmes-Farley]

Anyone else want to try before I give the answer?

 

[Kyuss414]

I'll go with D 2.6%

 

[Randy Holmes-Farley]

And the answer is...D. 2.6% more (as many of you got correct!)

The question was

"Compared to freshwater, the same volume of natural seawater weighs approximately:"

As with many of our questions, there are a variety of different ways to work this out. The easiest is using specific gravity, as chrisfraser05 pointed out.

The specific gravity of seawater (or anything liquid or solid) is the ratio of the density of the seawater to the density of fresh water. Since the volume is specified to be the same, the ratio of weight is the same as the ratio of density, which is the specific gravity.

So to answer this question, we do not actually need to know the density of seawater or fresh water, but just the specific gravity.

The specific gravity of typical seawater (35 ppt) is about 1.0264, or 1.026 rounded off.

So, the seawater weighs 1.026 times as much as the fresh water.

If the fresh water weighed 1.000 pound, the seawater would weigh 1.026 pounds, which is 2.6% higher than 1.000 pound.


As to whether the other answers are theoretically possible...


A. 35% more

The water would have a specific gravity of 1.35, which corresponds to a salinity of about 464 ppt. Nowhere is the ocean that saline.

B. 3.5% more.

The water would have a specific gravity of 1.035, which corresponds to a salinity of about 46 ppt. Nowhere is the normal ocean that saline. The most saline end of the Red Sea is about 41 ppt. Some tide pools are probably that saline at times, and trapped seas (like the Dead Sea) can be much more saline.

C. 26% more

The water would have a specific gravity of 1.26, which corresponds to a salinity of about 345 ppt. Nowhere is the ocean that saline, but this is close to the salinity of the Dead Sea. FWIW, the Dead Sea has different chemistry than the ocean, and you couldn't just make seawater that concentrated without some things precipitating (such as calcium carbonate).

E. 1.026% more

The water would have a specific gravity of 1.01026, which corresponds to a salinity of about 13.6 ppt. That corresponds to brackish water, like in an estuary where a river is delivering fresh water and mixes with seawater.

F. 1.020% more

The water would have a specific gravity of 1.01020, which corresponds to a salinity of about 13.5 ppt. That corresponds to brackish water, like in an estuary where a river is delivering fresh water and mixes with seawater.

 

[beaslbob]

checking checking

 

[beaslbob]

d

2.6% more means the sg is 1.026

Hot doggie

I got one right. :xd:

 

[Randy Holmes-Farley]

Congratulations to you and the others who got it right, even those who didn't post an answer, but knew it.