The left-handed Chawla, who came into bat at No 9 with the scoreboard reading 211 for seven, struck a stylish 112 off only 170 balls with 12 boundaries and four sixes as Somerset finished at 449 in 136 overs.
He reached his century off 139 balls with 10 fours and four sixes -- three off them coming against left-arm spinner Ravi Patel. The other six was hit off offie Oliver Rayner's delivery soon after he arrived at the crease.
Coming to bat in the 66th over, Chawla was finally out in the 130th over having guided his team to safety.
En route his three-figure mark, he shared a 107-run stand for the eighth wicket with Alex Barrow (65, 123 balls, 6x4) and 99 runs for the ninth wicket with Alfonso Thomas (54, 150 balls, 8x4).