Learn Dutch: Ik weeg honderd gram boter. - I weigh one hundred grams of butter.

In Dutch, when you have a specific quantity, you place the unit directly next to the noun. You do not use a preposition like van (of). So, "100 grams of butter" is simply honderd gram boter.

Why is gram singular here instead of plural?

In Dutch, units of measurement and weight (like gram, kilo, meter, and liter) stay in their singular form when they are used right after a number greater than one.

The current lesson is about the quantitative er. Why isn't er used in this sentence?

Quantitative er is only used to replace a noun that you are counting or measuring. Because the noun is explicitly named in this sentence (boter), we don't need to use er.

How would I use er to just say "I am weighing 100 grams of it"?

If someone already mentioned the butter, you can leave out the word boter. When you drop the noun but keep the quantity, you must use er to replace the missing noun. The sentence then becomes Ik weeg er honderd gram.