The mempool and the Merkle root don’t have anything to do with each other.

Blocks encompass a set of transactions chosen by the miner (most likely from their mempool, however it does not actually matter – the composition of blocks is miners’ perogative). That block consists of a listing of transactions, and a block header that incorporates amongst different issues a Merkle root committing to these transactions.

When blocks get propagated, nodes confirm its Merkle root towards its set of transactions. The receiver’s mempool is fully irrelevant on this strategy of deciding whether or not or not the obtained block is legitimate. As soon as a brand new block is accepted, transactions within the receiver’s mempool which had been included within the block (or battle with transactions within the block) are eliminated.

Now, some block relay protocols (notably, BIP152 “compact blocks”) take advance of the truth that receivers usually have most if not all transactions in a block already. BIP152 particularly simply sends a brief hash of every block transaction relatively than the complete contents instantly, letting the receiver request lacking transactions if any. However none of this has something to do with the block’s Merkle root – that is a side of validating the block as soon as it’s obtained (and reconstructed). Compact blocks is a way of transmitting the block earlier than validation occurs.