To determine the results for white flowers, first let us have a look at the genotype of the F1 offspring being crosses with the white flower.
When a true breeding purple plant will be crossed with a true breeding white plant, the offsprings produced will all be heterozygous for purple colour. This result can be depicted from the following punnet square:
p p
P Pp Pp
P Pp Pp
When a heterozygous purple plant will be crosses with a white flower, the results of the cross will be :
p p
P Pp Pp
p pp pp
The results show that about 50% of the plants produced will have white flowers.