Vtyjkyuk

Now I am writing a scientific paper, and I want to write a sentence as follows:

A has a chemical composition so similar to that of B that it also has a high potential.

As you can see, the first 'that' is used to substitute for 'chemical composition', and the second 'that' is used to introduce the following clause.

However, I am so worried because it seems to confusing due to the repetition of 'that'. (Because I am not a native, I cannot make a sentence with assurance...)

So, I tried to use a comma before the second 'that', but I don't know if it will be a correct sentence.

There are times when a comma may be added for the sake of readability. For example:

Those that know, do. Those that understand, teach.

                                 â€“ Aristotle

However, I think your sentence reads fine and needs no comma. Moreover, sentences that contain the word that two times are not uncommon. As a matter of fact, sometimes the two that's appear consecutively:

Don't be seduced into thinking that that which does not make a profit is without value.

                                 â€“ Arthur Miller

If you still insist your sentence seems awkward, here's what I might do instead of trying to fix it with a comma:

A also has a high potential, because it has a chemical composition so similar to that of B.

SUPPLEMENTAL to J.R.'s answer:

A comma would not be appropriate before the second that: this that marks the complement of so ... and consequently should not be disjoined from it.

As J.R. remarks, the double use of that is not really problematic. It's really two different words, demonstrative that and subordinator that. They are only historically and orthographically the "same" word. In Present-day English speech they aren't even pronounced the same way: the demonstrative is always stressed, and the subordinator is always de-stressed and pronounced with a reduced vowel.

I confess, however, that I personally find the use of two orthographic ‹that›s in different senses a little jarring in written English when they're so near one another. My own solution would be to recast the sentence to eliminate the first—perhaps something like this:

A's chemical composition is so similar to B's that it also has a high potential.

Your instinct to use a comma before the second that is a good one.

Commas do not have hard and fast rules, and as J.R. points out, there are indeed times when a comma may be added for the sake of readability.

I, however, respectfully disagree with J.R.'s opinion about the use of a comma before your second that. I think that doing so makes your sentence clearer, and more easily understood. And that is the point of good writing; not being so obsessed with rules like "Don't start a sentence with 'And'", and "Don't use a comma unless it's absolutely necessary".

My rule is this: If a comma helps me to convey meaning, and if it helps my writing sound more conversational, I almost always use one.