To find the number of students who have chocolate ice cream, we first need to determine how many students have vanilla ice cream.
Given that 19 students have vanilla ice cream out of a total of 73 students, we can find the number of students who have chocolate by subtracting the number of vanilla ice cream students from the total number of students:
\[ \text{Number of students with chocolate} = \text{Total students} - \text{Students with vanilla} \] \[ \text{Number of students with chocolate} = 73 - 19 = 54 \]
Now, we want to find the ratio of the number of students who have chocolate ice cream to the total number of students. This ratio can be expressed as:
\[ \text{Ratio} = \text{Number of students with chocolate} : \text{Total number of students} \] \[ \text{Ratio} = 54 : 73 \]
Thus, the ratio of the number of students who have chocolate ice cream to the total number of students is:
\[ \boxed{54:73} \]