Apple doesn't appear to currently publish the weight of its keyboards - noting that with release of the new models of iPad Air M3 and iPad, much of the previously listed technical information and the iPad comparison page have now been inexplicably (and unhelpfully) removed from Apple's website.
You might try contacting Apple Support and asking the question directly. You can contact the Support Team using the Support link at top-right or bottom-left of this page.
Alternatively, you can initiate and manage your support cases from Apple devices using the excellent Apple Support App. If not already installed, the App can be downloaded from the App Store:
https://apps.apple.com/app/apple-support/id1130498044
You might also visit the Genius Bar at your local Apple Store or Apple Authorised Service Provider (AASP) - where the technicians may be able to offer assistance:
Genius Bar Reservation and Apple Support Options - Apple