Learn Italian: Il sacco di patate è molto pesante. - The sack of potatoes is very heavy.

When describing the typical contents of a container, Italian uses di without a definite article. Just like un bicchiere di vino (a glass of wine), a sack of potatoes is un sacco di patate. Using delle here would sound like you are talking about a sack holding a specific, previously mentioned group of potatoes, rather than just describing the type of sack.

Pesante ends in an "-e". Is it a feminine adjective?

No, pesante belongs to a group of adjectives that end in -e in the singular for both masculine and feminine nouns. Since il sacco is masculine singular, pesante fits perfectly. If it were a feminine noun, it would still be pesante. To make these adjectives plural, you always change the -e to an -i (for example, i sacchi pesanti).

Does molto change its ending here to match il sacco?

No, because in this sentence molto means "very" and is acting as an adverb describing the adjective pesante. Adverbs never change their endings. Molto only changes to match a noun (like molti, molta, molte) when it means "a lot of" or "many" and is attached directly to a noun.

What is the difference between un sacco and ?