To calculate the percent yield, you need to follow these steps:

1. Calculate the experimental yield by dividing the actual yield (9.0 grams) by the theoretical yield (10.0 grams) and multiplying by 100.

```

Experimental yield = (Actual yield / Theoretical yield) x 100

Experimental yield = (9.0 grams / 10.0 grams) x 100

Experimental yield = 90%

```

Therefore, the percent yield for this reaction is 90%.