Here's how to determine the element with the largest percent by mass in Cu₂OH₂CO₃:

1. Calculate the molar mass of the compound:

* Cu: 63.55 g/mol (x 2 = 127.10 g/mol)

* O: 16.00 g/mol (x 3 = 48.00 g/mol)

* H: 1.01 g/mol (x 2 = 2.02 g/mol)

* C: 12.01 g/mol

Total molar mass = 127.10 + 48.00 + 2.02 + 12.01 = 189.13 g/mol

2. Calculate the percent by mass of each element:

* Cu: (127.10 g/mol / 189.13 g/mol) x 100% = 67.2%

* O: (48.00 g/mol / 189.13 g/mol) x 100% = 25.4%

* H: (2.02 g/mol / 189.13 g/mol) x 100% = 1.1%

* C: (12.01 g/mol / 189.13 g/mol) x 100% = 6.3%

Conclusion:

Copper (Cu) has the largest percent by mass (67.2%) in the compound Cu₂OH₂CO₃.