This is a bit tricky, depending on how accurate result you need.

As a first approximation - if you dissolve some amount of substance into 500 mL and you take 1 mL of the solution, it is effectively as if you took just 1/500 of the initial mass of the substance (so 81,53g/500) - then just divide by the final volume to find concentration.

In reality dissolving 81,53g of KNO₃ in 500 mL of water will produce more than 500 mL of the solution (finding out exact volume requires using density tables, in this particular case it will be around 530 mL), so you are not taking 1/500 but a bit less.

That being said the way we work here is: show the numbers you have gotten so far and we will start from there.

Hello,

I have some difficulty calculating values after dissolving a substance. I've gotten different answers from my environment, but I'm not sure who is calculating it the right way. I hope someone here can shed some light on the issue.

We dissolve 81,53g of KN03 in 500ml of demineralised water. We then dilute 1ml of the solution into 100L of water. In the end, what will the ppm of K and N be in the 100L of water? What is the correct way to calculate these values?

This is really too broad of a question to answer in any useful way. The most widely used tools for molecular identification in an analytical lab would be mass spectrometry (MS), possibly interfaced with some form of chromatography. But there are dozens of varieties of MS, each one suited for particular types of analyses. The particular method you would use in any given circumstance would be specific to the type of chemical, the matrix, the precision/accuracy requirements, the likely concentration, and so on. Supplemental methods might be needed also depending on what information you need and how much information you are starting with. There are few occasions where a single method will do everything you need to do.

It's also worth pointing out that amateurs and students often assume that modern chemistry is a field where the analyst is presented with a blob of unknown stuff and tasked with determining what it is made of. In reality, though, this is not what most chemists do on a daily basis. Although some fields certainly require chemists to engage in detective work starting from a blank or nearly blank state, in most cases analytical chemists have an idea (or know exactly) what they are looking for, and most problems are more centered around sorting out the signal from the noise (which involves both chemical purification and managing analytical interferences), measuring accurate concentrations, or confirming that they do indeed have what they think they have. The modern chemical toolset is oriented accordingly around targeted rather than black box analysis.

Yes, that's exactly the problem - but in the end it depends on what you consider a valid substituent. Once you assume isopropyl is an acceptable substituent you don't have to worry about its internal structure.

It should be 4,5-diethyl-6-methylhept-3ene.

"Isopropyl" is one of these names that are still acceptable, but not preferred. 4-ethyl-5-isopropyl-3-heptene clearly and unambiguously describes the molecule, so using it won't produce serious problems in communication with other chemists (but can be disallowed in formal context of publication).